What would cause 1999 Chevy Z71 350 Vortek 4 wheel drive to have a loud knocking sound when first started after sitting for a long period of time and stops after warmed up Oil is changed regularly?

maby one of the crankshaft jornals or main or rod bearings have a little more wear than normal . don't worry unless it knocks after it warms up run thick oil 30wt in summer 20-50 in winter will help cushin the gap between bearing and jornal keep your RPMs no higher than 3500 youll be fine for years to come ; you can ask for Steve at auto zone in lake city ga