5 Reasons Why a Car Makes a Knocking Noise When Driving Over Bumps

Updated on January 3, 2019
eddiecarrara profile image

Eddie spent 35 years in the automotive business with Honda. He is an ASE Certified Master Technician and has bruised knuckles to prove it.

Is Your Car's Knocking Noise Visible or Hidden?

If your car is making a knocking noise when driving over bumps, here are five components to look at before you bring it in to your mechanic.

Before we dive into the diagnosis, I need to let you know that not all noises are easy to find. Some are hidden in internal components that aren't easily visible, for example struts and steering racks. Video and pictures tell a thousand words and I will be using them throughout this article.

1. Steering Rack Noise

Steering rack end bushings can wear out and cause a knocking noise. If you suspect your steering rack is causing a knocking noise while driving on uneven road surfaces here is one way to check for play in the steering rack end bushings.

Jack up the front of your vehicle and support it under the frame with jack stands. Then grab the front tire at nine o'clock and three o'clock, give it a shake from side to side, and have someone keep an eye on the inner tie rod. If you notice any unusual movement, or you feel a knocking in the tire, you may need to peel back the steering rack boot and check for damage, movement, or wear.

A sure sign of a rack end bushing failure is fluid leaking from the steering rack boot located at the inner tie rod. If you take a peek at the boot and you notice it wet with fluid, most likely the rack end seal is leaking because of play or movement in the rack end bushing.

Leaking steering rack end seal.  The leak is starting to seep through the boot.
Leaking steering rack end seal. The leak is starting to seep through the boot. | Source
Another view of a leaking steering rack end seal, starting to seep through the boot.
Another view of a leaking steering rack end seal, starting to seep through the boot. | Source
Leaking steering rack end seal. The possible cause is that the rack end bushing has too much play.
Leaking steering rack end seal. The possible cause is that the rack end bushing has too much play. | Source

2. Broken Sway Bar Links

Broken sway bar links are very common in most vehicles, because sway bar links take a lot of abuse and are not very rugged. The sway bar link is a little rod, built like a shoulder joint in the human body but with a ball and socket joint on each end. See the sway bar link near the beginning of the video below. Each end of the link has a nylon bushing ball socket with a steel ball inserted into it that has a threaded end.

The fix for this is to replace the sway bar link and hardware. It’s not a very difficult repair unless the sway bar link is in a very tight area like over one of the subframes. If that’s the case it may take a little longer to dig out the broken pieces.

3. Worn Sway Bar Bushings

Sway bar bushings are not part of the sway bar links; the bushings I’m talking about are located under the brackets that hold the sway bar to the frame or body of the vehicle. These small inexpensive bushings wear out over time and can cause a knocking noise while driving over small cracks and bumps in the pavement.

If you suspect this may be your problem and you would like to do a quick check, have a friend sit in the vehicle and close the door and listen for the noise. You stay outside the vehicle, grab the roof right above the driver's door, and rock the vehicle from side to side like you are trying to roll it up on its side. If the person inside the vehicle can hear the noise, your sway bar bushings will need to be replaced.

Replacing sway bar bushings is an easy fix unless they are located on top of a subframe or hidden away. Many bushings and brackets are easy to get to and only require the removal of two bolts on each bracket.

A Cool Tool to Diagnose Noises

Lisle 52750 Stethoscope Kit
Lisle 52750 Stethoscope Kit

The automotive stethoscope is a great tool for identifying the location of a noise in your vehicle. I have one in my tool box at work and at home. They save you time and money because you can pinpoint the noise location quickly and you won't change unnecessary parts hoping the noise will go away.

 

4. Leaking Struts or Worn Strut Bushings

Worn or damaged struts can make a clunking or thumping noise when you ride over bumps and large cracks in the road. Leaking struts are easy to diagnose if the leak is external, but if the leaks are internal you’ll need to do a bounce test to the vehicle to know if the seals are blown internally.

The bounce test is just that: bouncing the car up and down and then releasing it to bounce on its own until it stops. Once you let go of the vehicle, it should not bounce more than three times on its own. If it does, you’ll need to replace the struts, and I recommend replacing them in pairs.

If you inspect the struts and you notice a lot of fluid and dirt collected on the strut, like in the picture below, it's time to replace the struts, and again, I recommend replacing them in pairs, for equal ride quality on both sides of the suspension. But if money is tight, you could just replace the leaking strut until you have the money to replace the second one.

Strut bushings should be inspected visually. The strut will have to be removed to inspect the top bushings, but if the bottom of the strut has bushings, they should be easy to locate and inspect.

Shock absorbers are inspected the same way; most shock absorbers have a lower and upper bushing and are usually easier to remove than struts.

Oil leaking from the strut. If all the oil leaks out, this strut will start to make a knocking noise over bumps and cause excessive tire chop.
Oil leaking from the strut. If all the oil leaks out, this strut will start to make a knocking noise over bumps and cause excessive tire chop. | Source

5. Worn Ball Joints

There are several ball joints in a vehicle, some located in the front suspension and some in the rear suspension. Ball joints are another ball-and-socket combination: a nylon socket with a steel ball. The nylon socket usually wears out first, usually because of lack of lubrication or because of abuse, like from potholes.

There are several types of ball joints and they are not all created equal. Some ball joints are held in by clips, others are pressed in or bolted in. They all work the same way: by allowing the suspension to move and turn easily, until the ball joint becomes worn and noisy.

Not all ball joints are checked in the same manner when testing them for damage or play. Some need to be suspended with no pressure, others need to be persuaded with a large pry bar. I recommend either having them checked professionally or looking up the proper inspection method for your vehicle in one of the maintenance books or YouTube videos.

Knocking Noises From Other Car Components

You probably have guessed by now that there are a lot of other components that can cause banging, rattling, and knocking noises, I just covered the five most common ones above.

In the pictures below you find some other components that are also often the culprit for these types of noises: hopefully point you in the right direction to finding out what is causing the noises in your vehicle.

Mounts

These are motor mounts that have failed. When a motor mount fails it can cause all kinds of knocking noises because nothing is holding the motor to the frame and the engine is flopping around in the engine bay. There's at least three more mounts.
These are motor mounts that have failed. When a motor mount fails it can cause all kinds of knocking noises because nothing is holding the motor to the frame and the engine is flopping around in the engine bay. There's at least three more mounts. | Source
This is a strut mount that is missing the rubber bushings, it was hitting metal to metal on every bump.
This is a strut mount that is missing the rubber bushings, it was hitting metal to metal on every bump. | Source
This is a motor mount that has collapsed internally. The mount holder (top) was hitting metal to metal every time the motor moved, making a loud knocking noise.
This is a motor mount that has collapsed internally. The mount holder (top) was hitting metal to metal every time the motor moved, making a loud knocking noise. | Source
Here is a transmission mount that has failed. Part of the mount was hitting the frame of the vehicle and was making a clunking noise on every bump and start off.
Here is a transmission mount that has failed. Part of the mount was hitting the frame of the vehicle and was making a clunking noise on every bump and start off. | Source

Rusted Rotor

The brake rotor in this picture is rusted to the point that the metal is starting to flake off. Every time the customer would apply the brakes it would make a thumping noise.
The brake rotor in this picture is rusted to the point that the metal is starting to flake off. Every time the customer would apply the brakes it would make a thumping noise. | Source

Broken Axle Shaft

Here is a broken axle shaft, as the shaft started to break it was hitting the lower control arm making a horrible banging noise.
Here is a broken axle shaft, as the shaft started to break it was hitting the lower control arm making a horrible banging noise. | Source

Compliance Bushing

This is a compliance bushing filled with oil, the oil is used as a cushion against vibrations. Once all the fluid leaks out it can start to make a clunking noise.
This is a compliance bushing filled with oil, the oil is used as a cushion against vibrations. Once all the fluid leaks out it can start to make a clunking noise. | Source

CV Joint

This is the internal guts of an inboard CV joint, it was lacking grease and failed prematurely and was causing a nocking noise and vibration under heavy acceleration.
This is the internal guts of an inboard CV joint, it was lacking grease and failed prematurely and was causing a nocking noise and vibration under heavy acceleration. | Source

Ice!

This chunk of ice was jammed against the driveshaft, every time the car started to move it would make a loud thumping noise against the floors.
This chunk of ice was jammed against the driveshaft, every time the car started to move it would make a loud thumping noise against the floors. | Source

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

If you have a question or if I left something out, leave me some feedback and I’ll get you the answers you're looking for.

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    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      5 months ago from New Hampshire

      Hi Dolores,

      I recommend driving it for a few weeks and check to see if your oil level goes up in the crankcase. The liquid dripping from the exhaust is most likely condensation, not gas (which is normal). If gas was exiting out of the exhaust system, your car would be running extremely rough, how's she running?

    • profile image

      Dolores sears 

      5 months ago

      I have a 1986 Camaro 305 5.0 T P I fuel injection Camaro IROC-Zs and after changing my oil I discovered gas in my crank case. I just put new fuel pump in her and changed her plugs started her up again and found gas after walking to rear of car dripping underside from exhaust. Now I did put dual exhaust on her Bad idea could that have caused this gas in my oil problem?

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      It's not a bad idea Liz lol.

    • Eurofile profile image

      Liz Westwood 

      8 months ago from UK

      That's interesting. I guess it is easily overlooked. Maybe I should check next time I have a service.

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      Hi Zia,

      Well... you shouldn't use straight water, it should always be a 50/50 mix of antifreeze and water. The water will push out of the top of the radiator and if you're lucky you'll notice the temperature gauge moving towards the red/hot zone and shut off your engine. As long as the engine didn't overheat you should be ok, if it overheated, you could have blown a head gasket or warped the cylinder head.

    • aziza786 profile image

      Zia Uddin 

      8 months ago from UK

      What happens if someone forgets to put the radiator cap back on after topping up water?

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      Hi Liz,

      That happens more than you would think. One of our mechanics did the same thing on the first oil change to a customer, the customer stopped at another Honda Dealer because they smelled something burning after we changed the oil and found our oil cap in the same position you found yours. The dealer called us and explained to us what we did wrong so we paid for all the cleanup and another oil change. Their mechanic, unfortunately, did the same thing, left the oil cap off. By this time, the customer was not so sure the problem was fixed because they still smelled something burning. The customer called us again and wanted us to check to see if everything is ok and we found the oil cap on the top of the engine not installed again. You're not alone Liz lol.

    • Eurofile profile image

      Liz Westwood 

      8 months ago from UK

      Thanks Eddie. I think my travel knowledge is minor compared to your car expertise. I shall be checking out your hubs next time I have a problem with our car. So far so good. Although I was defnitely unimpressed after the first service. By chance we lifted the bonnet a 100 miles or so later to check the screenwash, only to find the oil cap sitting on top of the engine, as the garage had forgotten to put it back on after changing the oil!

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      Hi Liz,

      Thanks, if I ever visit Porto, I'll have all your Hubs downloaded on my phone, it looks like you have extensive knowledge about the area :)

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      Hi Alex,

      If a shop just did the work I highly recommend you bring it back so they can check their work. Without hearing the noise it's tough to tell what is causing it. There are several components that could cause a knocking noise like connecting rod bearings, crankshaft bearings, timing chain/belt tensioner, worn cam lobe, etc...

    • profile image

      Alex matthew 

      8 months ago

      I have a Nissan almera just change the head&block, but whenever I start-up I hear a knocking in the engine what can that be?

    • Eurofile profile image

      Liz Westwood 

      8 months ago from UK

      These are useful tips as ever. I once had a knocking noise from a stone trapped in the wheel.

    • eddiecarrara profile imageAUTHOR

      Eddie Carrara 

      8 months ago from New Hampshire

      Hi Zia,

      Yes, when an axle snaps the car will not move and you'll need to apply the parking brake to stop the car from rolling, putting it in gear or park will not stop the car from rolling free. Thanks for stopping by and taking the time to comment, I really appreciate it. If you ever need any automotive advice, you know where to find me :)

    • aziza786 profile image

      Zia Uddin 

      8 months ago from UK

      Very useful information on knocking noises when driving over speed bumps. As a Uber driver, these car problems are a nightmare for me, especially when im carrying 4 passengers and driving over the bumps and potholes. The experience is terrible because of the wear and tear on my suspensions, bushes, wishbones and droplinks.

      Last June, my left side driveshaft snapped and my car didnt move at all even in gear and with the clutch released. There were no noises to indicate this will happen because the driveshaft was corroded in the middle due to contact with water.

      Thanks for sharing this hub, it may come in handy in the future.

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