Probably what you heard is the releasing of pressure building up in the crankcase, the lower part of the engine. This happens when the PCV valve or part of the system clogs. This type of fault is likely to blow oil seals. So make sure to keep an eye on the oil level and check the front and back of the engine for potential leaks if you see a drop in oil level. If you've never serviced the PCV system, it'll be a good idea to check the engine for sludge buildup. Internally, this is a very corrosive substance that can damage internal components.