When you have the any of the rear windows down and have the front windows up, the wind pressure enters the cabin and then creates a pulsating pressure within the cabin. This is due to the pressure waves colliding and cancelling or amplifying each other. Similar to when two cars have sports exhaust systems and cruise next to each other. When the exhaust pulses are in harmony, you can hear the....MmmmMMMmmmMMmMMMmmmm noise. That's the sound waves cancelling and amplifying each other. The reason why it doesn't really happen to older cars, is because they are not as 'air tight' as new cars. Old cars generally didn't seal very well or the seals are already old and wind escapes easily. Of course, being newer cars, they are designed better ( <_< ) and should be more air tight. This air tightness helps to reduce the amount of 'energy' (fuel, electrics, etc) to run such things as air-con. It would be very inefficient to have an air conditioner in a pergola. Works better in a room, right? :) Hope this helps. -Chris