Hopefully this is informational if your E9x M3 6MT (or DCT) is making new weird sounds. Sorry for the long article but I wanted to be clear.
This week was somewhat unpleasant. At ~11.5K miles, on last Saturday, the car started to make weird "popping" sounds from the back. The sounds would happen sometime during a shift, after a shift or just decelerating/coasting/engine braking between 2-3K RPM. This was not the usual clutch/flywheel judder/shuddering sound that this performance car is supposed to make.
I took the car in on Tuesday and was unable to duplicate the sound the on a test drive with a chief mechanic at the dealership. After driving the car, they recommend a diff fluid change to some new compound based on some BMW SIB (under warranty). On Tuesday evening, the car seems a lot more smoother to shift/drive. The sound no longer shows up. Diff fluid magic?
Wednesday: Car not driven
Thursday: At the end of a 50 mile trip (with 2 stops), 1 mile away from my garage, the car suddenly makes a loud grinding/metallic/holy-S$!t sound. It is like taking 2 metal garbage can lids and rubbing them against each other at high rotational velocities; coming from the back of the car. Car will shift, drive silently at slow speeds. Scary-ass-transmission-sounds-screwed sound at 2-3K RPM especially when coasting. I though the transmission was a goner.
Friday: I take the car in at 8AM. I haven't seen 8AM in like 6 months!
Sound stops on the 1.5 mile drive to the dealership from my home. On a test drive with the shop foreman, unable to duplicate the sound again. WTF - felt like I was losing my mind and hearing things. I leave the car with the foreman and let him drive it all day while I was at work. On the 4th drive, (he seemed to have driven the car 10-15 miles that day), he was able to make the car make that sound after a hard turn.
After raising the car on the lift, they notice that the left hand side differential bolt has broken through clean:
(Part M12X100-10.9 on the left hand side)
According to the techs/shop foreman: either the bolt was torqued incorrectly or the metal was bad(das german engineering quality control?). They even showed me the broken bolt later when I picked up the car (much appreciated).
The diff bolt was replaced after removing and remounting the diff, torquing everything back there correctly. They overnighted the part (2 dollar bolt!) and I had the car on Saturday afternoon. Quick!
As a side effect, the "judder"/shudder, 2nd gear notchy feeling and occasional feeling of shifts/clutch not being smooth have gone away. The clutch feels a lot lighter, the shift sounds are almost gone. The car feels new and different and just smooth overall! The dealership seems to have done an awesome job, took the time to explain things to me. It seems all the other parts around are not damaged due to the broken bolt. Time will tell.
It seems some other E9x owners and some E46 owners have had differential mounting bolt issues. Sucks but I guess its a performance car and stuff breaks.