I have an '01 with 135k miles that I have owned since new and is essentially stock. I have replaced diff and trans fluids every 50k miles with RedLine Synthetics. In addition to normal driving, I participate in ~2 HPDE events/year.

I have developed a noise when cruising at ~70 mph that sounds like a flute. Under load (maintaining speed, going uphill) it goes away, under no load (coasting downhill), it goes away. It's been getting louder and is now hard to have a conversation at highway speeds. I thought it might be the differential, so I changed the fluid early; made no difference.

There's never been any noise from the diff when cornering (no popping at low-speed tight cornering), the tires don't have any cupping, and the wheel bearings seem OK, as there's no movement at either 3 and 9 or 12 and 6.

With the high mileage, and the frequency of rear-end hum caused by the differential, this would seem plausible. But it's more like a "hoooooooo" than a "Hummmmm". Before I pull the differential and look at the ring and pinion gears, any other ideas?

 

Had the same set of symptoms with my 02 with 90,000 miles. It was the diff. If you replace suggest you check Gene Culley's price or get a rebuild if there is someone competent in your area.

 

Any chance it could be a wheel bearing?

 

The key there is "someone competent". Otherwise you might end up with the same problem and doing it again. Ask me how I know.

 

I take it the humming occurs under light load while driving on a level highway and you can make it come and go by slightly moving your foot up and down on the throttle? If so, that is a clear symptom of a diff issue. However, I would check the bearings more thoroughly than you have. They can make noise without being visibly loose when testing them the way you did. The fronts are easiest to check. First, take the car for a drive and listen to the hum as you zig zag back and forth on the highway. If the hum changes as you change direction that may indicate one or more bad hubs. Second, remove the front wheels and brake pads and lock down the rotors with a couple of lug nuts. Then turn the rotors by hand and listen for any sounds, notice how much effort it takes to turn them and notice if you feel any roughness as they turn. If you don't notice anything take off the rotors and turn the hubs being alert to any roughness you can feel with your fingers. Buying a new Timken hub from Autozone or other parts store will give you a reference point to how your hubs should feel. The rears are harder to check since you have to remove them from the car.

I had a lot of growling/humming on my 97 that I thought was coming from the diff. When it was checked out the mechanic found the right front hub was rough when turned by hand. The noise reduced considerably but was still fairly loud. It turned out all 4 hubs had to be replaced. After the repair it was amazing how quiet the car got on the highway even when running the GY run flats.

Bill

 

I have the same issue on a rebuilt 390 Diff. My vote goes to the diff not bearings but check it out