Im no expert, but if it goes away when you let off the gas, I'd believe it to be a ring & pinion issue. My gearset howls like an old hound, but I was cheap at the time, and bought a used pumpkin out of a truck.... You get what you pay for. If yours is a newer set that you had set up, I'd suspect it wasn't set up spot on.

If its worn like that for a while, I'm not sure if you can go back and adjust it or not. There are a few guys on the board here that are good with rear end questions.... I'm sure someone that knows whats up will chime in.