I think a bearing in my rear diff might be going bad. I get this humming sound linked to vehicule speed that increases with speed. It sounds to be centered to the rear. Both rear wheels bearings are under 2 yo, so I suspect something else. How can I make sure the rear diff is the cause ? What would be the best way to diagnose this ? I was thinking of putting the car on all 4 jack stands and trying to rotate the wheels with the engine running in first gear but I don't think that it's the safest idea haha
Thanks

It's really hard to discern where a noise like that is coming from, as a bad wheel bearing (either side) will send lots of noise / vibration through the axles into the differential / body (steel conducts sound VERY well).

I just bought one of these for the express purpose of chasing down a problem like yours...

https://www.ebay.com/itm/125589911587

Full disclosure - I haven't used it yet, though did power it up and it's very pretty. ;-)

You could clip microphones on the diff (one near the pinion and maybe one on the main housing) and one on each knuckle, and maybe one on each brake caliper (taking care to keep the cables away from getting caught in moving bits).

Or if that's too much like work, try going around sweeping corners (where you're really putting a side-load on the wheel bearings). If it's a wheel bearing, the noise will change. If it's not, it won't.

There is no change in sound when cornering, that's why I suspect the rear diff :P

Have you been able to check the CSB (Center Support Bearing) on the rear propshaft? That'll sound like it's coming from the rear(ish) of the car...

It could also be that, that's why I'm asking for tips :P

I do think putting a microphone on the suspected part would be a good idea. I did rig one of those up, using a cheap clip-on contact mic (designed for a guitar pick-up, IIRC), and plugged it into a cheap battery-powered FM transmitter. Then I just tuned the car radio to the transmitter frequency, and could listen to the part in question. There are a lot of ways to skin that particular cat of course, but my favorite (a mechanic's stethoscope) won't work unless you can strap yourself under the car and have someone else drive...

Put it on 4 jacks or a lift or have somebody put it in drive and accelerate. The noise location should become very obvious. have you done rear diff flush? Can drain and check fluid.

Here's a video I made

It sure sounds like the rear differential, but without a contact mic / mechanic's stethoscope to verify, you'd be taking a chance. As long as the car is firmly and safely supported by jackstands, AND the front wheels are blocked (unless it's an -xi...) it shouldn't be TOO dangerous to crawl under there and poke a stethoscope against the diff housing to make 100% certain where the problem is. I think it would be very obvious with that much noise being generated by something.

When was the last time the rear diff fluid was drained/refilled?

Not during my ownership (3 years) and I'm not sure about the previous owner but the chances are low. The car has 155k miles.

I would certainly try new fluid before throwing parts at it. You might just be low. Then again, if you've been low for awhile, it's possible the inner gears might have damage by now.

Either way, it's cheapest to start with new fluid. Make sure to open the fill plug first, before you open the drain plug. I think the rear diff takes about 1L of 75w-90 gear oil before it starts running out of the fill hole.

After you drain the fluid out, stir it up with a magnet and see what you get. If the differential is making noise chances are the pinion bearings are starting to go. Fresh oil might extend its life, but eventually it's going to get worse.

I think it takes 1.2L to fill.

The noise has been increasing in volume for the past year and there is no sign of leakage. Like you said, pretty sure that they are shot and need replacing.

Could it also be the Drive Shaft Flex Joint ? Or it makes a different sound ? I forgot to take a look at it when I was there.

Thanks

Finally had the rear diff rebuilt by professionnals after I realized that I didn't have the huge pliers required to remove the bearings circlips. The oil was still in good condition and there were no metal shavings.

I'm not happy to report that the noise is still there and that the diff wasn't the issue. Back to the diagnostic stage...

If the oil was in "good" condition it was changed as some point. After 90k miles, I would not call mine in good condition it was super black.

It could be u joints in the axles. Could be the main shaft.

Can you identify a bit better where the noise is coming from?

It's difficult by myself to identify the noise, even when the car is on jacks (also it doesn't like to spin it's wheels freely even with the traction control off)

Could be one axle joint yes, but I did not see anything out of order or any slack when I removed the rear diff.

my 2012 GLK 350 I also have a noise/vibration coming from the rear, put it on my indies, hoist, couldn’t really find anything, but a bent rim.
how do you get it to rev up, 2000 rpm, all sorts of dash messages? Problem occurs around 110/120 kph. Hear and feel it the most on deceleration as it passes though those speeds. Also hard steer to left more noise than right.

Greetings! Did you ever figure out what was causing that noise? I am trying to chase down the exact same noise on a 2014 GLK350 4matic. I've changed front and rear passenger side bearings, but the noise remains.

Yes, it wasn't the rear diff in the end, but a bad wheel bearing. If I had bought a stetoscope earlier, I think that I would have spot it.