Did you ever figure out what this was? And, did yours only do it when the windows and sunroof were completely closed and stop when you open the windows or sunroof just a bit? My 2007 Base is having the same issue but it starts around 60 mph and goes to about 80 mph (haven't had it faster than that on the highway.) I was told it could be a wheel bearing, transmission mount, or plastic cowl on the windshield...