The only real possibilities for this would be either the wheel speed sensors, or the relay for your ABS. It would most likely be the wheel speed sensor. You should bring it to your local dealer, have them diagnose to figure out which sensor it is. There are sensors on every wheels brake assembly, only one should go bad at a time. Once they tell you which one is bad, walk out and call Autozone or some other local car parts store. BMW wants $90 a piece, whereas the other stores only want about $30. It is a very easy install. All you have to do is unplug your old one and then plug in the new one.