Try removing the fan belt and running just the engine, without the fan, water pump, and charging system noises creating any distractions. It will run for a minute or so with the water pump disabled without overheating. I've seen square bearings in the water pump make people think the engine is about to hand grenade itself, and a noisy fuel pump can sound like a rod knock or a piston slap at times. Loose flywheel bolts can also make some oddball noises.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!