A rear differential is a box with a pair of bevel gears that turns the power flow through ninety degrees and shares it between the rear wheels. The teeth in it work with constant sliding and live in an oil bath, so wear here is almost always heard first and felt later. The classic handwriting is a whine that appears in a particular band of speeds and sounds different on power and on the overrun. That is what separates it from every other source of noise at the rear.
Why the whine changes with the throttle
When the engine is pulling, the teeth of the pinion press on one working face of the crown wheel’s teeth. When the driver lifts off and the car coasts, the load changes sign and the teeth bear on the opposite faces. If the wear is uneven or the backlash in the mesh is set wrongly, those two faces sound different — hence the signature effect: at 70 km/h on power it whines, at the same 70 km/h coasting it is nearly silent, or the other way round.
The second source of whine inside the differential is the shaft bearings. They give a steadier hum that depends less on the pedal but rises with speed. Often both sounds overlap, and by ear you get a layered whine with different shades at different speeds.
Telling it from other sources
| Source | Reaction to power and overrun | Reaction to manoeuvres |
|---|---|---|
| Differential, gear mesh | Clearly different on power and coasting | Almost none |
| Differential bearings | Weak, rises with speed | Almost none |
| Wheel bearing | None | Changes noticeably in bends |
| Propshaft centre bearing | Weak | None |
| Tyres | None | Changes with the surface |
The practical procedure is simple: find the speed at which the whine is clearest, and at that speed hold the throttle, lift off, and weave gently left and right in turn. A difference with the pedal points at the differential, a difference in the bends at the hub, and the absence of both usually means the propshaft centre bearing or the tyres — signs of a worn wheel bearing and signs of a worn propshaft centre bearing. The full catalogue of this family of noises is on the symptom page humming while driving, and the gearbox version of the same logic in gearbox whine at a particular speed.
And if instead of an even whine you hear a chattering click in turns, see signs of a worn outer CV joint.
What a workshop checks
A differential diagnosis starts with the simple things: the level and condition of the oil. Dark oil with metal particles in suspension and a bristle of shavings on the magnetic plug point directly at worn teeth or bearings. The seals are inspected at the same time: weeping means some of the oil has gone and the unit has been running short.
Next the differential is listened to on a lift with the wheels hanging, and the play is checked — end float in the pinion and backlash in the mesh. The result decides what is enough: an oil and seal change, adjusting the backlash and replacing bearings, or rebuilding the unit completely. On many models a used complete differential turns out to be the more economical answer.
How the repair is decided
The logical order is a cheap diagnosis with an oil inspection first, and only then a decision about the scale of the work — the cost of the differential repair itself depends heavily on the model and on whether the answer is bearings or a gear set, so it is quoted after the strip-down.
Two things mechanics do not always say. A crown wheel and pinion are a matched pair and are replaced together, never singly — a quote that offers one of them is a quote that comes back. And after any work inside, the backlash and the contact pattern have to be set with marking compound rather than by feel: a differential assembled “by ear” whines again within months, and that is the commonest reason a rebuild fails.
If the whine is still quiet and you want to confirm that it really does change with the throttle, record two short clips in the Pro-Stuk app: one on power, one coasting, at the same speed. The app matches the recordings with your answers to a few questions and shows the likely causes with percentages.
