I have encountered this plenty
I can almost gaurentee its your door seals. Clean and wipe them off real good and the surfaces where they ride. Once cleaned real well, they need to be treated well with a silicone based lube. Silicone dieletric grease would be great if it were a bit thinner/runnier. My experiance, silicone lube spray sucks for this. The absolute best thing i have found for this is BMW Gummi-Pflege. It is made spicificly for rubber seal treatment but no longer avalible less you get lucky and find some at a stealer or ebay, etc. BMW replaced it with the tiny $40 a tube, Carbaflow. I would try Wurth rubber care stick.
http://www.autogeek.net/wurth-rubber-care.html
That should do fine. Any goopy silicone treatment should do ok! Just make sure to clean good and work the treatment all over the seals and folds, etc