I had the exact same issue. They changed my rear diff fluid and eventually it went away. Took about 500 miles. This has happened to a lot of people on here. If it does not go away, I strongly encourage you to bring it back to dealer.
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