I've had two E39 540's (still own one of them) and now an X5....Both E39's had the dirty sock smell problem with the AC and it seems to be fungus growing on the condenser. Here is the only thing that worked in both 540's which should be similar in the X5. I've tried everything else and this really worked. http://www.bmwtips.com/tipsntricks/AC/smell.htm