That's the sound of air trying to get in and out at the same time. Same thing happens if you crack one window on certain cars. Maybe just the back window, or maybe just the front window. Has to do with the random shape/volume of the car, and how it reacts to the wind while driving, combined with the venturi effect of the passing wind trying to suck air out of the car.
Unless I misunderstood the question, LOL.