The short version
- The throttle separates them. A differential answers to load through the gears, a bearing does not.
- Lift off the throttle at a steady speed. If the tone changes, it is not a wheel bearing.
- A differential whine usually changes pitch on and off power. A bearing carries on regardless.
- Cornering separates them the other way, because a bearing responds to load transfer.
Telling a differential or wheel bearing noise apart matters because the two repairs are nothing alike in cost or complexity.
Both produce a continuous sound that rises with road speed, which is why they get confused. One test splits them.
The throttle test
Find a quiet road, hold a steady speed where the noise is clear, and change nothing except the accelerator.
- Hold light steady throttle and listen to the tone.
- Lift off gently to coasting, keeping the same speed as long as possible.
- Feed the power back on gradually and listen again.
Differential gears are loaded one way on power and the other way on overrun. That reverses which tooth flanks are in contact, and the sound changes with it.
A wheel bearing has no idea whether you are driving or coasting. It only knows how fast it is turning and how much weight it carries.
Reading the result
| What happens | Most likely |
|---|---|
| Pitch or tone changes on and off power | Differential or driveline |
| Unchanged through the throttle test | Wheel bearing or tires |
| Only present under power | Differential, loaded side |
| Only present on overrun | Differential, coast side |
| Changes on a gentle cornering sweep | Wheel bearing |
| Clunk taking up drive, then quiet | Driveline backlash, not a bearing |
The cornering half of that table is the load transfer test, and it is on the left or right page.
What each one sounds like
A differential usually whines. A relatively pure tone, often described as a siren or a howl, and frequently worse in one specific speed band.
A wheel bearing growls or drones. Broader and rougher, without the tonal purity of gear noise.
A worn pinion bearing can whine at a much higher pitch, because it turns several times faster than the wheels.
That last one is a useful tell. A noise whose pitch is clearly higher than wheel speed cannot be a wheel bearing.
Where the noise is coming from
Location helps less than people expect when separating a differential or wheel bearing, but it is not useless.
Differential noise is centered. It comes from beneath the middle of a rear drive car, or from the transaxle on a front drive one.
Bearing noise seems to come from a corner, although it travels through the structure and often points at the wrong one.
On a front drive car the differential lives inside the transaxle, right next to the front wheel bearings.
So the sounds originate inches apart, and location tells you almost nothing. The throttle test still works.
Other driveline sources
Several components sit between the engine and the wheels and can produce speed related noise.
A carrier or center bearing on a two piece driveshaft, which hums with road speed and often vibrates through the floor.
A transfer case on four wheel drive vehicles, which can whine and may change when engaging or disengaging drive.
A worn CV joint, which clicks on lock rather than humming. That comparison is on the CV axle page.
The full list of things mistaken for hub noise is on the confusables page.
Ruling the bearing in or out physically
The road tests narrow it down. Getting the car up settles it.
Rock each wheel at twelve and six, then at three and nine. A differential problem produces no play at the wheel.
Then spin each wheel by hand and listen at the hub. Roughness you can feel through the tire is a bearing. A differential will not do that with the wheel raised.
With one wheel raised on an open differential, running the drivetrain spins that wheel at double speed and the car is only held by stands.
Spin it by hand with the vehicle in neutral instead. The test works just as well and nothing is being driven.
The full checking sequence is on the checking guide.
Why it matters commercially
A differential or wheel bearing repair are not comparable jobs, which is why guessing is expensive.
A wheel bearing is a corner job with a known part. A differential can mean a rebuild, a gear set, or a replacement unit, usually at several times the cost.
What a bearing costs by comparison is on the cost overview.
Common questions
Is it a differential or wheel bearing noise?
Use the throttle. A differential changes tone between power and coasting, while a wheel bearing sounds identical either way.
Why does the throttle separate them?
Because gears load different tooth flanks on power and on overrun, which changes the sound. A bearing does not care whether you are driving or coasting.
What does differential or wheel bearing noise sound like?
A differential usually whines with a fairly pure tone. A bearing growls or drones, broader and rougher.
Does location help?
A little on a rear drive car, where the differential is centered. On front wheel drive it sits beside the front bearings, so location tells you almost nothing.
What if the pitch is much higher than wheel speed?
Then it is not a wheel bearing. A pinion bearing turns several times faster than the wheels and whines accordingly.
How do I confirm differential or wheel bearing noise on stands?
Rock each wheel for play and spin it by hand listening at the hub. A differential fault produces neither play nor roughness at the wheel.
Can I run it in gear on stands to test?
Do not. On an open differential a single raised wheel spins at double speed with the car on stands. Spin it by hand in neutral instead.