Every once in a while regardless of speed, I can start to hear a lound wind flapping sound inside the cabin. Its the same soud as when you only have two windows open. I have no idea what it could be and is starting to drive me nuts. I haven't taken it to the dealer yet because of the spontaneous nature of the sound and not being able to reproduce on command. Has anyone else experienced something similar?

Well I figured maybe that sound could be related to tires, but I just had a set of tires installed recently and same problem. I only tend to notice it on the highway, especially if its windy. Its noticeable enough to make your head hurt if you listen to it for a while with the radio off.

I think the noise your describing is produced as a result of differential air pressure in the cabin from the open window (s).

To resolve the problem readjust window opening slightly or open or close a 2nd window. If the noise disappears, you solved the mystery.

Could be bad weatherstripping?

I wish it were that easy. All windows/sunroof are closed.

Wash your car and see if any wet spots develop.