BMW Repair: Re BMW 528i smell of burnt rubber from the vents, bmw 528i 1998, valve cover gasket

Question
I have a BMW 528i 1998, I get a smell of burnt rubber when I stop the car once it has been running for a while. Not always there, it seems to depend on which way the breeze is blowing, quite strong at times. I have asked the guys who service the car to have a look but they can find anything wrong? Been going on for about 18 months now? Any ideas?

Answer
I have to start with the most basic questions. Have the microfilters been changed recently? Are there any fluid leaks getting on the exhaust (in particular the valve cover gasket)?  Also is the same smell coming from under the hood? Any rubber or plastic on the exhaust system at any point? Please let me know any answers you can give.
                 Kimber