Tranny ( I think ) noise

[Northern Ninja]

I have an 08 Sierra 2500, 6.0, 6 spd with around 80,000 miles. At around 40 - 50 mph when I let off the gas, I get a whirring/whining noise. It goes away as soon as I get back on the gas. Doesn't seem to matter if the truck is warm or cold, or If i'm towing or empty. The truck shifts solid and has no other issues. Any ideas where I can start looking? Thanks for the help.

[gmtech4]

diffs make that noise when they are "not right",,,,have it road tested by a tech or provide an audio of noise ,otherwise we are guessing also ,,,,waste of time right?