I have a 97 305 Vortec that has done this for a long time now, don't know what it is, but it sounds a bit like a lifter tapping, it goes away once warmed up and my truck has 200,000 miles on it, and its probably made this noise for at least half of that.Make sure you don't deviate from the oil type eg:5W30 or 10W30 or whatever the manual states, as a thicker cold oil will make this sound louder too.