Sounds like a water pump. Have it looked at asap, it's not anything you want to put off if it's failing.

g/l

__________________
'08 335xi (E90), 6MT, Arctic/Black, ZPP, ZSP (18"s), ZCW, CA, PDC, i-Pod
Ordered: March 7, 2008
Built: March 17, 2008
At port: March 19, 2008
Shipped on Graceful Leader: March 21, 2008
Arrived Port Hueneme: April 11, 2008
At dealer's: April 19, 2008
Delivered: April 21, 2008