This seems odd. I had the differential problem too. It would "creak" on tight turns such as parking lot spirals. They swapped out the differential fluid for "special" differential fluid. This took 45 mins by the way. There is a service bulletin on it. I was told the same thing, I needed to drive 600 miles after the fluid swap before it would stop. It eventually went away in a about 400 miles. I was never told that the 600 miles must me turning miles.
2011 E93 - Space Gray - 6MT/Silver Novillo/
OE Tuning/Gruppe M Intake/ACS Springs/Vorsteiner CF Type I Rear Diffuser/Textured Matte Black Custom Powdercoated OEM 359s/Painted Reflectors/Matte Grille, Hood Vents and Side Markers/Arqray Race Exhaust/MS Pulley