I took my Z4 to complain about the rattle sound of plastic on plastic. Sometimes it sounded like it was coming from the passenger seat area, and at other times sounded like it was coming from somewhere in the area where the rear window rests, or from the speaker area behind the driver's seat. No matter where it originated, it ended up sounding right in my ear. It was a high tone "click, click, click" more pronounced on rough roads.
After two attempts, the dealership found the problem in "clips" and replaced them. They gave me the ones causing the problem, and they were broken in half (I think I threw them away, however). Replacing the "clips" fixed the click-rattle, which had been driving me crazy. P.S., the clips were small, not even filling the palm of your hand, and were solid black. The tech who discovered the problem said he had "heard" about these clips failing (perhaps due to the roof movement up and down).