Differential Whine
Acceleration vs Deceleration

Every hypoid gear set has two sides to each tooth. Under throttle the pinion drives the ring gear and loads the drive side. Off throttle the wheels drive the pinion and the coast side takes the load. A noise that only shows up under one of those loads is telling you which side of the teeth, and which bearings, are unhappy.

The Road Test: Drive, Float, Coast

On a smooth, quiet road with the radio off, get to the speed where the noise is worst and run three load conditions on purpose:

• Drive. Light to moderate acceleration, holding steady throttle load. This loads the drive side.
• Float. Just enough throttle to hold speed with almost no load on the gears.
• Coast. Foot off the pedal, transmission in gear, letting engine braking slow the vehicle. This loads the coast side.

Note which condition makes the noise, the speed range, and whether it is a pitched whine or a dull growl.

What Each Pattern Usually Means

Whine on acceleration only

Drive side trouble. On an axle that has never been opened, this is usually a worn ring and pinion, often after years of low or broken-down oil. On an axle that was recently regeared, it usually means the pinion depth or backlash was set wrong, so the drive side contact pattern is sitting too far toward the top of the tooth, the root, or one end.

Whine on deceleration or coast only

The classic sign of loose pinion bearing preload. When load reverses, a pinion with end play shifts away from the ring gear and the mesh goes noisy. A crush sleeve that has relaxed, a pinion nut that backed off, or worn pinion bearings are the usual causes. Grab the yoke and push it in and out. Any play you can feel is too much. A leaking pinion seal often comes with it.

Whine both on drive and coast

Usually a worn gear set, a setup with no decent pattern on either side, or a pinion bearing that is coming apart.

Whine only at one speed range

A howl at, say, 45 to 55 mph that fades above and below is often gear mesh lining up with a body resonance, usually from a marginal contact pattern. Tires and driveline can do it too, so do the rule-outs first.

Growl or rumble that does not change with load

Gears change with load. Bearings mostly do not. A constant rumble that rises with road speed regardless of throttle is a carrier bearing, an axle bearing, or a wheel bearing. If it gets louder when you swerve and load one side, think wheel or axle bearing first.

Whine after a regear

New gears need a break-in per the gear maker: short drives with cool-downs, no towing early, and a fluid change around 500 miles. A whine still there after that means the setup is off. See our ring and pinion backlash and pinion depth setup guide.

Check the Fluid Before Anything Else

Low or burned oil causes most of these noises. Pull the fill plug: oil should be right at the bottom of the hole with the vehicle level. On the magnet, fine gray paste is normal; chips or bearing cage pieces are not. If the oil is low, top it off with the grade your axle tag calls for (add limited slip additive if required) and road test again. A quart-bottle fluid transfer pump ($19.99) makes filling through the side hole easy. If the noise quiets down, find the leak; the gears may already be damaged. Our differential fluid change interval guide covers how often to change it.

Rule Out Everything Else

• Transmission. At the noisy speed, shift into a different gear or slip into neutral and coast. If the pitch follows engine RPM, it is upstream of the driveshaft. If it follows road speed, it is the axle or driveline. Our guide to transmission whine by engine RPM vs road speed goes deeper, as does bad transmission vs bad differential noise.
• Tires. Cupped or aggressive tread hums like gear whine but barely changes with throttle. Feel for scalloping or rotate the tires and retest.
• Wheel bearings. A growl that changes when you swerve left and right at speed is a wheel or axle bearing, not the ring and pinion. See wheel bearing noise symptoms and cost.
• Driveshaft. Dry U-joints, a worn center support bearing or bad angles cause load-sensitive vibration and chirps. Check the joints for play.

With the axle on stands, a stethoscope such as the Steelman EngineEAR II ($186.99) on the pinion snout and each carrier bearing area separates pinion from carrier bearing noise. Stay clear of spinning parts.

Reading the Gear Contact Pattern

Drain, pull the cover, paint several clean ring gear teeth with marking compound, and roll the gear both ways with some drag on it. Then read it:

• Good pattern. Drive side centered between top and root, leaning slightly toward the toe. Coast side reasonably centered.
• Pattern high toward the top of the tooth. Pinion depth is too shallow. Move the pinion toward the ring gear centerline, usually a thicker pinion head shim.
• Pattern low toward the root. Pinion is too deep. It needs to move away from the centerline.
• Pattern toward the heel or toe. Mostly a backlash issue. More backlash moves the drive pattern toward the heel, less moves it toward the toe.

Check backlash with a dial indicator and magnetic base ($95.99) on a ring gear tooth at several spots. Many axles land somewhere around 0.006 to 0.010 inch, but use the number in your service manual or the gear maker's instructions. A big swing in backlash around the gear points to runout from a bent carrier or debris under the ring gear.

What It Costs: DIY vs Shop

A bearing job only makes sense if the teeth are clean. Pitted or chipped gears mean new gears, always with new bearings.

Parts and Tools for the Job

Gear oil and limited slip additive are in our fluids section. If the road test pointed at the transmission instead, CoreTransmissionParts.com carries rebuild parts.

FAQ

Why does my differential only whine when I let off the gas?

Whine on coast loads the coast side of the gear teeth. The most common cause is loose pinion bearing preload, often from a crushed sleeve that has relaxed or worn pinion bearings, which lets the pinion move away from the ring gear when load reverses. Low fluid and a coast side pattern that was never checked after a regear are the next suspects.

Is a differential whine on acceleration dangerous?

A faint steady whine can go for a long time, but it means wear is happening. A whine that gets louder month to month, turns into a growl, or comes with a clunk or metal on the drain plug magnet should be fixed soon, because a failed pinion bearing can wreck the ring and pinion and lock the axle.

Is it normal for new gears to whine after a regear?

A slight whine during the first few hundred miles of break-in can quiet down. A clear whine that is still there after break-in and the first fluid change usually means the pinion depth or backlash is off, and it will not go away on its own.

How do I know if the whine is the differential or the transmission?

Hold a steady road speed and shift the transmission to a different gear or into neutral while coasting. If the pitch follows engine RPM, look at the transmission. If it follows road speed and changes with throttle load, it is in the axle or driveline.