This problem can't be a wheel bearing since it changes sounds between power and compression. Wheel bearings, axle bearings will be constant. Since it's there all the time but is worse when on compression, it won't be a differential pinion bearing because the problem is related to forward and reverse thrust. My best guess would be the center carrier bearing on you drive line. It's possible that the bearing is going bad and the noise is worst because the dust shield is rubbing against the bearing housing on compression which pushes the drive line forward.