Im going to assume this rear has been set up wrong from the factory and lived its life like this. Im going to attempt what rusty said and tighten the hell out of it and see what happens. When guessing everyone is agreeing it's not the axle bearings.
I put it on Jack stands and got the car up to 40 and it was quiet. Im guessing cause no load.
Not saying it never happened but I never saw one from the factory howl. Don't go crazy tightening that nut or you may strip the threads. It's supposed to be torqued to 150 ft. lbs if I remember right
