Sorry if this has been covered before!

I've had my '08 TL-S for a few months now, and I've noticed that when I'm driving at highway speeds in certain wind conditions I hear a hum or vibrating noise coming from the back seat area. Yesterday I had rear seat passenger who said he could feel the plastic housing around the third brake light vibrating when the noise was happening! I'm wondering if it has something to do with the XM radio antenna or not. The cover on the roof does not seem to be loose.

Any suggestions?