Shake Shake Shake

sounds like a knock to me but cell phone audio sux. start pulling plug wires one at a time to see if any DONT make a difference pulled, ie a dead cylinder. Laser thermometer on manifolds would not be accurate as IR header pipes. 18436572? Easy to cross 5 and 7. Forged crank 340s (what year you got?) were neutral balanced and .040 over replacement pistons would be weight matched to the STD ones. If you stuck with stock or OEM weight pistons (not Icon, Probe, JE, etc) I doubt the crank is out of balance. Unless your running a 360 balanced converter? Cast 340 balancers (big cast in warning on them) look unlike a symetrical neutral balancer, you'd know. Sometimes at idle, the advance plate will be pulled out of gap spec, then it will hit on 8 off idle as the plate moves. Just double check the advance plate arc while looking at it while applying vacuum to the pot.