Check out the post Hummer Humming. I posted it because I was experiencing the same thing. The dealer found a problem in the rear end. It was a ring and pinion as well as all of the bearings that had to be changed. They didn't have all of the parts so they put in a complete rear end from a new vehicle. There is no more humming when accelerating or when stopping. The problem is real, I would advise having it checked out.
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