Rod knock and fuel knock are engine sounds. An improper air/fuel ratio or spark timing causes fuel knock. It sounds like a metallic tapping or pinging that rises with engine speed. Rod knock is produced by worn engine connecting rod bearings. A loud knocking sound increases with engine speed and reduces when the throttle is withdrawn. Lack of lubrication or overloading bearings can cause premature connecting rod wear. Fuel knock and rod knock can severely damage your engine if left unchecked. If you hear either noise, take your car to repair.