Loose flywheel to torque convertor bolts usually make an irregular knock at or just above idle especially when the engine idle is rough. Most of the serious engine problems have a regular beat to the knock. Loose or cracked pistons usually knock when engine is cold and idling. Wrist pin knock is most often heard with engine hot and idling. Connecting rod knock can be best heard when the engine is accelerated and the RPM goes over its peak. Main bearing knock can be heard when driving and under load, sounding like a heavy rumble. Valve train noise at the rocker arms is more of a tapping noise. Rocker arms hitting the valve covers is usually a lighter tapping noise. Yes there are many other sources for engine noises. These are some of the most common heard knocks when the fan belt is removed.
