I've got a 350 in my '76 K5 Blazer that has a dull knock (not loud, but noticable if you know how an engine is supposed to sound) comming from the rear of the engine on the passanger side. I've read a lot on engines and rebuilding V8s but I have never got into one. My guess is that it is from piston slap (piston rocking in the cylinder). There is also a noticable pop (at idle) from the exhaust on the same side of the engine. This engine was supposed to have been rebuilt about 2-3 years ago. The engine runs fine, I just am wondering if this is cause for concern.