Its more of a constant slapping noise from the plastic rear window. What makes it worse is that I drive across the San Mateo bridge on my commute. Nasty crosswinds.

 

I only hear the plastic window flap when my window is open. Guess we don't have high enough winds down here. I find that the tension on the back window is different for every car. Some people have very loose windows, while others have very tight ones. Guess that's the kind of variances we get for buying a hand built car.