Getting the wind up

ON A warm day in my Toyota Yaris sedan, I partially lowered the rear windows for more air circulation. As the speed increased a drumming noise, as if from a badly balanced wheel or misaligned steering, became so loud that I pulled over and checked the tyres and wheels. There were no obvious faults so I continued at a slower speed. The noise was still there, but later, when I used the airconditioning and closed the windows, it disappeared. I have since checked this on another Yaris sedan we own, with the same results. The problem appears to be aerodynamic.