Here is some info on startup knock for the 1996-2003 Vortec 350 and 305 L31 and L30 engines.
http://home.comcast.net/~cliff_scott/99-06-01-003.pdf
It seems that incorrect main bearings were used in some trucks. I dont think this is your problem however. since your noise starts AFTER the truck has started to warmup rather than immediatley upon startup.
peace
Hog