Rear end problem help

Yep.
Its the last adjustment process to ensure proper pinion to ring gear adjustment when you rebuild a rear, especially if you replace the pinion bearing. Can't just tourque down the pinion and hope it works.
If its just slapped together, you may hear whirring, and howling. These noises are harmonic, caused by improper pinion depth, and hi speed revolutions.
That is why it is adjustable.
Paint the blue on the pinion, spin it around and see where the blue ends up on the ring gear.
Then dial the pinion nut, in or out by tightening or loosening. Wipe down the ring gear and check contact again as needed. There are many charts or tutorials depicting adjustment procedures. If you have a crush sleeve type, and need the pinion to move "out", you will need a new crush sleeve so the pinion remains tight and doesn't slap.
If you have shims, remove or add shims accordingly to maintain proper pinion depth.
Joe