Probably a bad pinion bearing. You must disassemble the diff and replace both inner and outer pinion bearings using a shop press. There are a total of 4 bearings. Two side carrier bearings, and two pinion bearings. Usually its one of the two pinion bearings that goes bad, not the side bearings.

here in the uk its called diff whine ...wear in the crown wheel and pinnion ..but your may not be the diff ..get the wheel bearings checked ...good test of wheel bearings ..the whine gets less when cornering ..if it does get lighter its not the diff

*Without actually seeing the wear pattern on the ring and pinion gears we can't tell you if the wear is unusual which will cause noise.

In any event the ring and pinion should be re-set-up by a professional. The carrier @ pinion bearings plus pinion crush sleeve may be worn also.

find transmission shop (ATRA.COM). might have a differential bearing going. Or turn the radio up

rear axle faulty