7 1/4 noise

Hey guys. My rear end is making some noise and I wondered if anyone could help me out.

The sound can't be heard all the time when driving but if I go above 50mph it starts making a "houling" sound. Sometimes louder sometimes quieter. If I let the car coast and let of the accelerator the sound goes away but returns if I touch the pedal.

The rear axel outer bearings have been changed not so long ago and the rear is topped with oil.

Any ideas on what can be the issue? Tips n tricks appreciated.