Loud knocking when letting off gas

[James Culp]

I have a 99 Silverado LS 5.3 new body style. I noticed a loud knocking noise coming from underneath the truck when I let off the gas. I thought it was the front drive shaft u joint but I took the drive shaft off and drove it and it still has the knocking noise. Changed cv axles both sides. Put new seals on the front diff. Brand new suspension. New control arms. And still can’t figure out what it is. Any guesses?!?!?!

[davester]

Did you check the rear driveshaft?  Can you put the rear wheels up on jackstands and try spinning up the rear wheels then let them slow down and see if the sound happens?