[Ol'Jim]

Absolutely nothing to worry about, guys.
When GM added the engine oil cooler to the C/K trucks (1989 or 90), it added about 5 feet of tubing that the oil pump has to pressurize before oil gets to the main bearings. What you are hearing is a deep knock, right? (About 3 or 4 knocks then silence)... Main bearings with temporary lack of oil pressure.
You can minimize it by using 5W30 weight engine oil, (preferably Valvoline). Switch to a heavier oil, even 10/30, and it gets worse. Stay with 5/30, just like GM says, and change your oil (and filter, ALWAYS), every 2500-3000 miles. Do this and you may never hear that sound again. Very common on 90 & newer trucks, Fords do it too, but their computer immediately brings the engine to a fast idle for a second, to speed the oil pump up. You still hear them, but it's many lighter knocks, instead of a few heavy ones like the GM's. Also, wait too long between oil changes, and your oil thickens up, which makes it start the knocking thing again. I just hit 150,000 miles on my 95, and it did that for years when I was using 10/30 oil (because I was towing heavy loads and wanted the higher pressure the heavier oil gives you). Switched back to 5/30 during each Winter and last summer when I stopped towing with this truck, and no more knocking. My 1990 was sold with 293,000 miles on it, same deal. Pulled a 10,000 # trailer with both.
Relax, there is nothing wrong with your truck! Enjoy the coming Summer, and change your oil!