the snapping and crackling could just be the catalist in cat cooling, i just had a hell of a chirping like your speaking of, it was periodic and or speradic, i looked while jacked up and saw nothing, jigled exhaust and it came from header to cat conection, couldn't tell till i had it on the lift and found it was the 2 bolt hanger on back of block(beside oil filter) had broken and was rubbing together, are you positive that's not it? would look fine until you acctually pull ex. down or unbolt bracket from block