I experienced the same sound out of my 216 a coupla years ago. Clunk sound at the lower part of engine at idle and disappear when the engine revved up. It was a bad harmonic balancer. Any rod or main bearing noise, and piston slap I experienced with other engines always got louder when revved up.
Good idea....how were you able to diagnose it? Will it have some play if I grab it and wiggle it with the engine stopped?
