I hope this doesn't pretain to your problem but I fought the same problem with a small block, 283, in one of my trucks in years past. I'd replace a lifter here and there trying to get rid of a slapping lifter sound and ended up with my father in law looking at it. Old time mechanic told me cracked piston around the wrist pin. I thought, you're crazy, how can you tell by just pulling a plug wire here and there. Experience, he was right, I had one piston come apart in my hands while removing it and the other was obviously cracked. The sound seamed to come from the lifter but didn't. I hope you find an easier solution.Larry
