if you had a worn in differential wear whine it WOULD NOT go away after a while no matter how long you drive. It will be more evident at certain speeds and you should be able to vary the sound by slightly accelerating and coasting. The only way to erase a rear diff whine is to replace the ring and pinion gears. Even new bearings and clutches will not erase a diff whine. Heavier weight synthetic fluid and a blanket or other sound proofing under the rear seat will mask it though
