loud knocking

i have a 87 b2600 carbed automatic. everything was working fine. i could actually drive it. i put a mechanical oil pressure and temp gauge in it to keep an eye on things. the other day i was driving home with no problems and then all the sudden when i let off the gas there is a very loud noise that comes from the motor. its kinda like a knock. but loud and fast. the truck still ran fine had oil pressure and was running cool. i drove it back to the house and let it set. cam out next day and started right up. little knock on start up but ran perfect. if i rev the engine above 2500 and then let off it knocks real bad again but it drives like a dream. i am stumped anything will help.

I had a similiar problem with an '86 B2000 "back in the day". Knocking was very minimal during start-up and at idle. It got louder or more noticeable when I let off the gas. It was also noticeable during steady cruising. If I was accelerating it was very hard to hear. If this sounds familiar, it might be the same. It turned out to be a spun rod bearing. Hope this helps you.........L8RS!!!