At freeway speeds, I'm getting this strange rattling/buzzing that I can't seem to pinpoint. It's coming from the front passenger door area, and almost seems like maybe 2 pieces of thin metal rubbing together, or maybe styrofoam? It's fairly high pitched.

It seems to be vibration induced rather than wind induced, as a particularly smooth freeway surface will quiet it down even at speed.

Any ideas on this? It's incredibly irritating and gives me a headache on longer drives!

2008 G35S Sedan.