Differential Noise?
 

The noise is a rumbling or growling that happens on very light throttle, when the driveline is just barely loaded. Any more or less throttle, the noise goes away. Is most prominent at highway speeds. It is not apparent at speeds less than 40 MPH. It's very difficult to pinpoint where it is coming from but seems like the rear. Sounds a little like knobby tires running over rough pavement. There is no vibration associated with the noise.

My initial thought was axles based on threads in this forum. Unfortunately, a pair of CVJ axles did not cure the noise, although my old axles leaked so replacement was not a waste. During axle replacement a new differential mount and Mobil 1 gear oil were added. Flex disks and center bearing are less than a year old.

I have a hard time imagining what else besides the differential could make the noise under those conditions. Wouldn't bearings or suspension parts be independant of throttle?

When I was chasing sounds in that area (not Mercedes) got the Rear Wheels up off of the Ground and the Car on Jack Stands with the front wheels blocked. I ran the Car in various gears while the wheels spun and I listened with in my case a Wooden Broom Stick.
By listening you might find that it is a Rear Wheel Bearing going out and not the
Differential.
In my case (on the Volvo) while polking around I also found my Support Bearing and a U-Joint needed replacement but part of the sound still is still there.
It was my Support Bearing that was making the Grinding Sound as that part of the noise is gone but I am left with a vague howeling sound when I get up some speed.
My listening with a stick showed there was nose coming from the Differential.

Wheel bearing noise is not normally load (acceleration ) dependent, Diff noise, normally crown wheel / pinion is, you can play a tune going off & on the gas with most diff noise.
Some diffs make a noise & can go on for years without failure. Some times the noise can be overcome by re setting up the diff. You need to know what you are doing to get a good outcome.
Some times the ware on the crown wheel & pinion is such that all the adjustment possible will not fix it.

2.88 diffs are notoriously short lived I think. I am on number two and am pretty sure the humming I am having is the diff.

What kind of oil is in the diff? I'm starting to think the synthetic stuff bubbles to easily.

Huh??? I've seen a fair share of 300K+ mile 2.88s still running around.

With no known service other than fluids? I am just basing my opinion off a number of posts and my own personal experiences. 2.88 seems to have a higher failure rate than the 3.07.

I had my differential cover off and I found it to be in pretty good shape. From the maintenence log the fluid was changed regularly.

I had my left rear clunk recently though like something wasn't grabbing. I was at a stop and turning left so maybe I'm doomed for failure soon.

wouldn`t a differential give a whine more than a clunk noise at speed?

Iam up to 359,200 on my 2:88.

Charlie

Oh I think your just hard on them, especially with your manual trans. You need to step up to the 1.3 liter case out of a gasser.



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Check the center bearing.

I will definitely check the center bearing again. I felt like the fit between the bearing and the carrier was a little too loose (no force required to seat the bearing in the carrier) when I replaced about 8K miles ago.

I had my Indy do the axle replacement and I left him with Mobil 1 75W-90. I asked him if he saw any metal bits when he changed the fluid and he said "no" and that things looked "OK" in there. Who knows, though, not having seen it myself. The diff. had a pretty pronounced whine since I've owned it (I've put about 15K on it) and the synthetic gear oil has decreased the whine substantially. However, the rumble seems to be a little more pronounced in that it is easier to get the car to make the noise. The comment about being able to play a tune with the throttle pedal is spot on.

I know differentials can go a long time while making noise but I always thought that applied to a whine. Is there any chance this thing is going to grenade? The noise is not terribly loud and I would like to wait a while on a diff swap. Is there anything else besides carrier bearing I should rule out? Can Diffs be rebuilt? Are 2.88 diffs with ABS hard to find?

The carrier and center bearing look fine. The flex disks are still good. Driveshaft looks fine. If suspension and bearing related noises are not load-sensitive that would seem to leave the differential or trans. I really don't think I have a trans. problem.

Anybody have any thoughts about adjusting/rebuilding differentials vs. finding a good used one? How long can a rumbling/growling diff. go before destruction? What happens when it self-destructs (i.e. is it likely to be dangerous or hazardous to other expensive parts like half shafts?)

Can you be more specific about this "rumble".
Do you get a clunk as you back off or go from no throttle to acceleration ~ backlash.
If there were no metal particles in the oil, one may think that its not too bad, a rumble sounds bad though and is not normal.
Do you get more noise when you go around bends?
Diffs can be rebuilt but I think it would be cheaper to replace the complete unit.

It's a tough noise to describe. Not really a grinding. Knobby tires on rough pavement is the closest I can come only there is no vibration. Can sound a little like and ABS system but, again, without the vibration and the "beats" are so close together as to be one noise. Cornering makes no difference. I am not able to hear the noise below 40 MPH. No clunks or clanks when accelerating or deccelerating. Although, if you rev the engine in park you can hear a slight clunk and see the driveshaft turning a slight amount indicating some play in the differential. No vibration or noise under heavy acceleration. Seems to have become worse since axle/diff. mount/diff fluid change. Can easily reproduce noise on highway by the right amount of peddle pressure (very slight). Letting off the accelerator completely or heavier pressure and the noise goes away.

I know exactly what you're talking about. I had these same symptoms and subsequently swapped out the diff on my '81 240d. The noise is still present, but slightly reduced. The old differential did not show wear and hardly had any play in it (perhaps 1/8"-1/4" driveshaft movement before the axles would engage).

While changing the differentials I also switched the axles, putting the passenger side on the drivers side and vice-versa as I was told this would lengthen their usable life...

My guess is the axles are the cause.