Automotive problem

When you are driving about 65mph or faster yu can roll down rear windows an hear this annoying bumping sounds which sounds likes wheels are out of balance. I have replaced tires and struts and had all balanced. Is it possible this could be an aero dynamics problem. You only hear this sound when rear windows are down not the front. I would greatly appreciate your input. I am 60 yrs. Old and never experienced anything like this.