I bought the Yakima WhispBar for my E91 along with their SprocketRocket fork mount bike rack and am a very happy camper. The Whispbar is very quiet compared to any of the previous roof mount systems I have used on other cars I have owned and pretty easy to take off and put back on with the roof rails on my 2010 E91. Not quite sure if they have adapters for the rails you will get on a 2012 E91 however.
AKA AlanL on all other forums
2005 X3 2.5i - Mystic Blue
2010 328ix Sports Wagon - Space Gray