the driving force that is applied to the ring gear by the pinion gear in the rear axle assy, while driving the car backward, is loading the opposite faces of the gear teeth from the forward driven gear faces...clearances for the front faces are the only ones used for adjustment, and whatever is left between the rear drive is often less than ideal and noise in reverse is common...most operators never even notice reverse gear whine due to low speed, short distance of use, or lack of sensitivity.
DO check the rear axle for sufficient lubricant, slight leakage there is practically impossible to stop and capacity is limited under the best circumstances...and check (believe it or not) power steering fluid level , low fluid in the pump will cause a ''whir'' which , strangely, often is noted in reverse drive long before it is audible in forward gears
