I too had this exact same problem in my 01 x5 3.0. I wanna say it occured around 80,000 miles give or take 10,000. I wish I could remember what the exact fix was, I want to say something about a regulator...I can't be certain but my mechanic knew right away what the issue was, replaced the parts and it never happened again. I'm at 183,000 and it has not reared its head. I'll try to see if he remembers since it was a few years ago, but i remember it not being too massive of a job. Hopefully i'll get back with a more exact answer