I get the same thing on my E46, had the differential oil changed once or twice, it's still there.
2015 SO/SO MT M3 :: Exec : Lighting : Adaptive : HK : CF trim : Full leather : DAP : Black 19's : sunshade
Crystalline tint 40%/70% on windshield : M performance CF mirrors/spoiler : RKP diffuser
Dechromed with gloss black badge, grilles, side markers
V1 Savvy hardwired : Self-coded, will code locally for beer or food