I just bought the same 2006 GMC Envoy SLT last week, and am experiencing the same problem. Everything was fine for the first 5 days, now I get the whining noise through the sound system. When I turn the radio down, there's no noise. When I turn it up just 1 small click, I hear a whining through the speakers and also a simliar sound originating from the fuse box (?) under the rear driver-side seat. Maybe something to do with the powered/amplified speakers is housed in the same area? Or maybe I'm just hearing the rear speakers whining.
Did you find any solution to the problem? Thanks.