Stech

I have a 2009 5.3 <NOBR style="FONT-FAMILY: verdana,geneva,lucida,'lucida grande',arial,helvetica,sans-serif; COLOR: darkgreen; FONT-SIZE: 100%; FONT-WEIGHT: normal" id=itxt_nobr_0_0>Silverado</NOBR> Z71, Since new I have a weird noise that I cant pin point exactly where it comes from.

When starting the truck after it has sit for 1 to 2 hours I get a semi loud knocking sound kind of like piston slap but just a little different. It lasts for approx 2 to 3 minutes. It is not there at all when starting from cold in the morning, or sitting for a long period of time, only after its been driven and then sitting for an hour or so. The dealership is stumped, they have heard it a few times now but just cant pin point it.
I seem to pick it up with a stethoscope the best at the oil pump or on the side of the block at the #1 cylinder area. But again it doesn't do it on first start in the AM.

Anyone ever hear of this or have any ideas.