Rumbling sound from differential (non-LSD)

[ScottAndrews]

This may be obvious to many, but the way the open diff works is that the ring gear is turned by the pinion (super obvious). Attached to the ring is a cage that holds four small gears (the spider gears-sometimes also referred to as spider and sun gears). When the speed of the two wheels is the same, the ring turns the cage and the spider gears simply turn the output shafts but don't themselves rotate. When the speed of the wheels is different (for example when turning in a circle), the spider gears rotate so that the average speed of the two wheels matches the speed of the ring gear, but one turns faster and the other slower.

Below is a video of this.

If you are getting noise when the wheels are turning at different speeds, then either the spider gears themselves, or the bearings for the spider gears are shot.

I'd recommend you change the diff oil and try driving it gingerly for a bit. It may just be that the gears and bearings need to be exercised a bit. If the noise doesn't go away, then the spider gears and/or bearings have probably gotten rusty. You may be able to get it rebuilt, but I would caution against doing the yourself. You need a fairly large press, some special tools, and a set of spacer shims. It is really easy to mess up the setup, and if you do you will have put a lot of effort into a rebuild and end up with a junk differential with the ring and pinion gear faces worn through.

 

[ScottAndrews]

I no differential specialist, i just like taking things apart, ; this is what i know:

Standard diffs have a few bearings that are available through Walloth and Neesch. Replacing the side bearings is easy, they can be pulled out without opening the diff.
It does need to have the half shafts dismounted for it. In my side bearing I found a split (broken) ball retaining ring:
View attachment 207097
The bearing of the pinion wheel (the one in the direction of the drive shaft) isn't easy to replace as it requires a precise pre-load setting to be achieved through a crush washer that needs a precise torque and friction measurement in order for the pinion gear to match to the diff perfectly. The workshop manuals ("blue books") contain a detailed procedure for it.

Catalog picture 33/02 - BMW 2,5 CS - 3,0 CSL (E9) - Rear axle | Wallothesch

www.wallothnesch.com

So, one way would be to replace the side bearings leaving the pinion bearing untouched. Fingers crossed; it solves your problem, then it's an "easy-out".
If it didn't solve the problem, then perhaps the pinion bearings or the gear teeth itself are creating the problem.

Here's my diff, pulled apart. I went for the "fingers crossed" option, keeping the pinion bearings and crush washer.
I also see that i bought a O ring from the dealer - check via RealOEM which bearings and seals parts there are, replace all seals.

View attachment 207096 bearings.

Erik; You cannot re-use the crush washer. You can reuse the shims if they are put in exactly where they came out, but you will need a torque wrench capable of measuring small torques (inch pounds here, I guess Newton-centimeters there). Before you put the ring gear in, you will torque the pinion nut a bit at a time until turning the pinion shaft requires some specified small torque.