The rear axle has a large ring gear and a relatively small(er) pinion gear. If their spacing isn't exactly correct their mesh will be "off" and they'll be noisy at a specific speed, usually upon either acceleration or deceleration. So if the noise always occurs at a certain road speed, and usually in a 2 or 3 mph range, then it's probably the ring and pinion mesh.
This can be corrected by adding or subtracting shims to move the pinion gear ever so slightly forward or backward for perfect mesh with the ring gear. There may be talk that the gears have already worn in and changing their mesh is not advisable - that depends on the car's mileage - because, yes they do have some wear now. The higher the car's mileage the less advisable it would be to change anything.