I have had 4.3`s on cold start would knock a bit when they 1st started up.Nothing serious just a minor knock enough that when I herd it I kknew that I had a problem.When warm you never know hear any noise what so ever.What I did is put some oil additive that quiets noisy engines and an oil change.Then I drove the engine lightly for about a year before it became so bad something had to be done.Also when the same thing as you mentioned when warm good oil pressure but eventualy I noticed I started I losing just a bit of oil pressure.In the winter where I live the cold can get harsh so eventualy I had to use a block heater.One day the engine wouldnt start for whatever reason so I was working on it.It was in G20 van.I took the doghouse off and thought I removed all the change mainly 1cent peaces.Well I heard something drop inside the throttle body and looked down inside.I noticed a couple of penny`s so I plucked them out except the one I missedLOL.Well it sucked through the intake and into the valves.OOPS!So much for that engine.Sorry I cant answer your question but thats just a little story about cold engine knocks that occur on these engines.

 

yeah, i dont know... ive got 275 or so ks on my truck and it just started doing that piston slap thing a few weeks ago. My stepdads half ton silverado (4.8l V8 [which he babies like nothing else!]) has done that for ages