That's probably the motor in your ABS module- pretty well known problem. It can be expensive if you buy a new unit (1,500+). I'd recommend taking it out of the car yourself and sending it to "Module Masters" to rebuild it, as that is what i did. It costs around $300 plus some other stuff you need (you need to bleed the brakes afterwards), but expect your car to be out of commission for 3 weeks. Here's a post on it: http://www.e90post.com/forums/showthread.php?t=600491
A forum community dedicated to BMW owners and enthusiasts. Come join the discussion about Bimmerfest events, production numbers, programming, performance, modifications, classifieds, troubleshooting, maintenance, and more! Bringing the BMW community together.