Actually, a better seal in the cabin will create a worse effect. The problem is that air pressure is rapidly increasing then dropping, causing the rumble. like I said, in a friend'* brand new car, it was TERRIBLE but fun to play with
It'* just the design of the cars, I'd imagine the angle at which the windows hit the oncoming air has a lot to do with the rate at which it floods into the car and whether or not it becomes trapped.
The air is blowing out of the car because of the windspeed outside, the lower air pressure outside sucks it out. Then all of the sudden your car has become somewhat of a vacuum. It fills this vacuum with air, which in turn gets blown out again. This creates the rippling pressure effect, and consequently, a rumble in the part of your body which senses pressure changes the best -- your eardrums.
And those air deflectors that you can put on the cars would help if the rears were designed as the fronts are. There is no deflector on the leading edge of the window in the rear, only the top. If there was one on the leading edge, depending on your speed and stuff, the air would likely pass right over the window at a distance large enough to not suck out any air inside the car.