I'm a little confused- - - -how were the main bearings checked without pulling the oil pan? I'd suggest disconnecting spark plug wires one at a time to try to isolate the noise to a particular cylinder. A severely damaged piston will also cause a noise, but the "clang" description isn't something that's usually associated with internal engine 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!