If it is the IHKA internal fan, remove the unit, take it apart and clean the internals and fan of all the dust accumulated there. That fan runs all the time until the car goes to sleep mode.

__________________
E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey