Update- my issue was also the rear diff. Metal shavings were found in the sample they took. BMW is going to replace the whole thing.
If you hear a soft knocking or chirp sound coming from the rear of the car or from under the rear seats, definitely recommend that you have a dealer check it out.
Ugh, now the wait begins to get my car back. In the meantime they gave me a 330, which feels slightly different than the M8
