after 60k miles and putting coolant in every year, I finally noticed the expansion tank had a crack on the seam.
CHECK your tanks, people that have been adding coolant.
Part was 150$ from the dealer, but takes 3 min to replace...dont forget bmw coolant only.
You can source a tank for 100 from bmw supply shops online but I was in dire need.
http://www.realoem.com/bmw/enUS/show...diagId=17_0322
