I had a hella bad noise when a coil went out. I was missing two cylinders and coil would go in and out. Was a bad connection. It made a crazy clunking snapping sound. Little backfiring... Ran perfect after fixing the wire. I realized it was only running on two during a cold start and tracked it down. Maybe the loose connection would make the coil fire off prematurely resulting in the piston slap. I don't know.