I have a 1999 GMC Suburban 2WD. There is a loud howling or whining noise that seems to be coming from in between the driver and pass seats to me but others have said it sounds like it is coming from the rear? It only happens when my foot is on the gas. At 40mph it is very loud. I have changed out the tranny fluid, diff fluid in rearend and the u-joints. At this point I can't tell if it is from the rearend or the tranny area. Any help would be appreciated. Truck functions fine, no problem with shifting or torque, just the loud howling. Thanks, Randy