When shifting my automatic transmission into reverse recently, I have noticed a low pitch sound like a fan is hitting something. The noise goes away in 15 to 30 seconds. Transmission not giving me any problem. Happened once before a few weeks ago.Was rear ended a few weeks ago and car was repaired by BMW.Problem appeared after repair.
Transmission problem? on 2006 BMW 325i
I would definitely take it back to the shop that repaired your BMW. There is issue called 'hidden damage' when a car is being repaired after and accident. This basically means that some cars have to be attended to more than once after accident to be completely repaired.