So while cruising around I hear this loud howling coming from the rearend. At first I thought wheel bearings? But then I realized it only happens in 4th gear around 2400-2600 rpms and in 5th gear from 1800-2000 rpms. When I had it in the air a couple of weeks ago the housing looked in good shape with no leaks or cracks. Anybody have any advice as to what it may be?