Real mystery - Mechanic stumped

[TMR013]

So, a couple weeks ago I took my 135 into my local independent with a right rear tire whine, he diagnosed a bad bearing and we replaced. I did inspect the old bearing and there was definitely signs of rust (makes sense we had some bad flooding last year, and I had to drive through some water that was pretty high). So I pick up the car and sure enough the whine is gone, but I drove about 2 blocks and I heard a new noise, kind of a grinding noise coming from the rear, definitely a rotating noise cause it has a frequency that changes with speed, starts at about 25 mph and fades in and out. I immediately turned around and drove back to take my mechanic for a ride. He heard it too and thought it might be the diff, said to bring it back and they will get it up on the lift and see if they can locate it.
So took the car back today, they heard the noise, pulled the axle apart again and tested the new bearing, said it was totally stable and noiseless, did not think that was it. He measure the rotors and did find a warp that was definitely out of spec. It made sense that it might be that since as I said the noise is something rotating, so we changed out the rear rotors. Put it back together, noise still there.
My mechanic could not figure it out. Still suspects the diff, but he checked the oil and did not see anything wrong there. He even crawled into the trunk through the rear seat while someone else drove to see if he could localize the sound.
This independent is well established and a very well reviewed BMW specialist. Been going to him for a couple years and always been satisfied. He only charged me the parts for the rotor replacement today, so I don't think he's trying to fleece me. He seems genuinely stumped. Going to talk to a guy he knows that's a BMW diff expert to see if he can help.
Anybody else ever run into something similar? It's really driving me nuts because it's not a sound that elicits confidence in the car, and I hate that I don't trust my Baby.

[ShocknAwe]

New sound with a new part, too good to be true. Something wasn't torqued down properly would be my first guess.

But you said they took it all apart and reassembled?

[lowbudgethero]

that would be my first guess

[TMR013]

Originally Posted by ShocknAwe

New sound with a new part, too good to be true. Something wasn't torqued down properly would be my first guess.

But you said they took it all apart and reassembled?

Yup, completely pulled the bearing again. Pretty sure these guys wouldn't make that kind of mistake twice

[EMWOLB]

A grinding noise is usually associated with gears. In your case, the ring and pinion. A roller bearing like for a wheel bearing would have a whine.

You probably need to have the 3rd member rebuilt. This isn't a difficult job, but it does require an experienced tech.

[dm874]

Is it an oem bearing?

[JimD]

I'm trying to remember what the rear end looks like. I believe it has a cover, like I am used to on American cars, but it has a brace or something attached to it. Is that right?

Anyway, if you are worried about the rear end it is probably time to pay him to pull the cover and look. That could be as simple as pulling a dozen bolts or is probably a bit harder with the cover attached to something with a few larger bolts. But it will come off. If the water got high enough to damage a wheel bearing it is likely to have gone in the differential too. There is a vent.

[ShocknAwe]

Originally Posted by JimD

I'm trying to remember what the rear end looks like. I believe it has a cover, like I am used to on American cars, but it has a brace or something attached to it. Is that right?

Anyway, if you are worried about the rear end it is probably time to pay him to pull the cover and look. That could be as simple as pulling a dozen bolts or is probably a bit harder with the cover attached to something with a few larger bolts. But it will come off. If the water got high enough to damage a wheel bearing it is likely to have gone in the differential too. There is a vent.

Diff has a rear cover, with I think 8 bolts, and a single use fill/drain plug.

On another note, how would water get into the diff? It's sealed.

[dtla1]

Just to make sure, have them check the handbrake position and adjust if necessary. It's probably not this, but could be the emergency brake shoe rubbing on something and making that noise. Simple check if it's not this, and won't take more than 15 minutes to adjust if necessary.

[TMR013]

Originally Posted by dm874

Is it an oem bearing?

Yes, this guy uses only OEM parts

[iminhell1]

Check a wheel speed sensor.
The 25mph thing would sound like the stability control is commanding a brake to be on. I don't know about BMW, but with other makes I've seen a sticking bearing cause the ABS/TCS to activate erratically.
It's as simple as throwing it on the scan tool and watching the sensors.

[TMR013]

Originally Posted by dtla1

Just to make sure, have them check the handbrake position and adjust if necessary. It's probably not this, but could be the emergency brake shoe rubbing on something and making that noise. Simple check if it's not this, and won't take more than 15 minutes to adjust if necessary.

Already done.

[MightyMouseTech]

Originally Posted by ShocknAwe

Diff has a rear cover, with I think 8 bolts, and a single use fill/drain plug.

On another note, how would water get into the diff? It's sealed.

Diff should have a vent.

[ShocknAwe]

Originally Posted by MightyMouseTech

Diff should have a vent.

the more you know.

[MightyMouseTech]

Originally Posted by ShocknAwe

the more you know.

Never seen a diff, transfer case or transmission without one. Otherwise the seals would blow out when it got hot.

[dtla1]

Any update?

[TMR013]

Originally Posted by dtla1

Any update?

Yeah, pretty sure it's the diff. Taking it back next week. My mechanic bought a new tool, actually a gizmo, that allows him to put microphones on various parts of the car and listen to them independently while it's on the road. That will verify if the noise is coming from the diff.
Already started the process to replace with a KMS LSD. So at least I will get an upgrade out of it.

[david in germany]

How old are your tires? My wife's 328i had a terrible sound in the front end for a year or so running the original Bridgestone Runflats. Had the dealer check the berrings, rotors etc. Once I swapped the tires out, quiet as a dream.

[TMR013]

So, to update everybody, after my guy bought his listening gizmo, we both decided that it was the diff so I went ahead and ordered a KMS Wavetec mech LSD, he put it on, and,,, the noise was still there.
Beyond frustration at this point. I told my guy to replace the wheel bearing he originally installed. He did, and guess what? NO NOISE! Somehow got a bad OEM bearing.
When it came to the bill, I knew I could be a real jerk about it, justifiably, and my mechanic offered to cover all the labor for all the work he did, but I figured I got a pretty good upgrade out of it. So I told him I would split the labor for the diff install, he covered everything else, and we settled.
Now, as far as the new LSD. I'll be honest, I could be imagining this to make me feel better for the $3K I put into it, but the car does feel, more active, best way I can describe it. Just seems like it's just slightly more responsive in curves and acceleration. I have about 500 miles in the new diff now and I haven't taken it out and really thrashed it yet, but I'm happy with the upgrade. Plus I like it that I now have a somewhat more unique 1er. If your thinking about it, I would recommend if you want to put down the Benjamins. But honestly the difference is really subtle, you have to pay attention to notice it. I'm sure if you track, it's much more apparent, and I can't wait to go out on a twisty.
Despite all the problems, I'm pretty happy how it worked out.

[KNS]

Thanks for the update. Sometimes a part is bad right out of the box.