This has been brought up so many times...it's normal. It has to do with the climate control system, from what I've heard, but every single E39 does it. Don't worry.
Anyone who ever buys used cars online, anyone even THINKING of starting a "Is this deal too good to be true?" thread, anyone who wants to be more educated about protecting themselves must <a href='http://www.carbuyingtips.com/fraud.htm' target='_blank'>read this</a>.
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