The term 'unwanted engine sound' refers to two distinct sounds that can be heard coming from an engine. The first is a 'KNOCK, ' which is a loud, sharp sound that is caused by the combustion of fuel in the engine's cylinders. The second is a 'PING, ' which is a high-pitched sound that is caused by the engine's valves not closing properly. Both of these sounds are undesirable and can indicate a problem with the engine.