I had a similar noise in my 2012 328i which was not there when the car was new but got progressively worse as the car aged. It was NEVER there with the A/C off and not always there when cold but got worse after a long drive. Mentioned it many times to the dealer who couldn't fix it. They re-pitched the fan at one time which seemed to help for awhile. Bought a new 328i in March 2015 which didn't have the noise when new but now 8 months later I have a suspicion it is developing in this one. Could be my imagination though, I will have to keep listening.