I too am having a noise when letting off the accelerator, usually when going down hill and over 35 MPH not when in town driving. It sounds like sticking a plastic straw in the spokes of a bike when the bike is turned up side down and pedal is turned really fast. It made the sound one time when reversing out of the driveway too. It started after putting the truck in 4-wheel drive to come off the mountain. I pulled over and took truck out of 4-wheel drive when I got to pavement and then when I picked up speed I heard the noise, acts like the transfer case is not disengaged all the way.

Update from Oct 18, 2010: Took my truck to the tranny place, and the guy did not even have to hear the noise to know it was in the transfercase, he said "$1200 if the case is cracked" (which it will do if you keep driving with the noise), and "$800 if not cracked". OUCH...anyway long story short, he opened up the transfercase, replaced a bent fork (cause of noise), a couple of bearings (messed up by the bent fork), and put in new seals...$550. Thank God for the honest repair man! Runs great and no more noise.