Noticed the other day that there's a lot of noise coming from the back when my foot is off the gas. It goes away when I put my foot on the gas and accelerate.

As far as what the noise sounds like, it's hard to say since I'm trying to drive and turn my head to hear what's going on back there, but it's like a whirring noise. I don't get any vibration - at least nothing I can feel in the driver's seat. Maybe a U Joint issue?

 

Does the noise happen when you let off the throttle?

Try jacking up the truck, spining the d-shaft and listning to the u-joints.

I had a rear u-joint go bad after a road trip. It started to squeak when the driveline unloaded and contacted the backside of the componants.

 

Check the pinion bearing, when they go bad they can make a loud whining sound.

 

Definitely only when I let off the throttle. The slightest touch of the gas and it goes away.

The kid that owned the truck before me never took care of it so I'm in the process of going through it and finding the things that need to be fixed. Lucky me!

 

Could be u-joints, or the pinion bearings are going bad and/or don't have enough preload.