Start checking where the whistle comes from by sitting in the rear while somebody else is driving. I have heard C classes whistle in the upper corner of the little window, but either the window or door rubber can cause it as well...
Start driving around and push/pull the parts of the door to check whether the sound increases or decreases.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)