What Causes "Howling" Rear Gears?

Wrong viscosity oil.
Someone before you or you yourself replaced the pinion or ring gear and not it's partner. Gears develope a wear in pattern over time and don't much like having one replaced and not the other. Bearings, lash and bearing preload, housing saddles are out of align to each other, some numskull used heat on the ring to carrier bolts to remove them causing a wobble. The hardness on the gear set is all wrong from new.