warning.....i did not read the whole thread, sorry if this has been ruled out. but,....
the chevrolet 5.3 is Notorious for rattling.. there is a TSB on it and it is deemed "normal" by GM they call it "piston slap" and say it is caused by the flat top pistons they use. my silverado with a 5.3 has done it on start up (especially when cold) for as long as ive owned it. i purchased it with 50,000 miles and have clocked another 100,000 on top of that. It sounds almost like a rod knocking and mine also has some valve noise too, which is another common complaint with the 5.3. i was told by a GM technician (who i trust) not to worry about the valve noise either unless it is running rough or missing, which it is not.
i hope this helps, and if it does not i hope you find your problem soon (and its cheep to fix)
here is some good info on what i was talking about.http://www.chevyavalanchefanclub.com/cafcna/index.php?topic=4450.0;wap2