Car Rear Window Pressure Wave

I think it's down to the pressure differential between atmospheric and the car interior. As the car travels forward, the movement of air around the open window creates a low-pressure environment within the car which is then filled with higher-pressure air from outside. I imagine this will only happen at speeds appropriate to creating a sufficient pressure variation. I've noticed it tends to occur when the sunroof is open too. I imagine the internal layout of the car has an influence on this too. Why it does'nt seem to happen when just the front windows are down I don't know, but this may have something to do with your position in the car?