See if anyone has the same experience as me, and I'm sure they have but I can't find anything in the forums.
If I only open one window in the back seat it sounds like a helicopter is in there until you open another window and the air can flow through. Theories have been just the design of the vehicle to air trapped in the speaker compartment.
So anyone have a tangable theory or know why it happens??

It happens to yours because it happens to both of mine. All cars do this to some extent.

try this: pop up your sunroof and crack the right rear window. now adjust the window to taste.

The wife'* Pont. Vibe has got to be the worst for this. It will suck and vibrate your inner ear right out....darn painful. I had the dealer head mech. in it for a ride concerning its steering, and since he was pissing me off I rolled down the back widow and left it down all the way back to the dealership. He claimed all the newer vehicles did this.

He said most people drive with their air on in the summer. Not me I told him. I like fresh air. He was visibly agitated. I enjoyed his pain!!!

Yeah my car does this a little, but a buddy of mine just got a Jeep Liberty Renegade... and that thing HURTS. We did it one day driving down the road and within seconds it felt like my brain was shaking against the sides of my skull.

my sister comments on it all the time, saying it hurts her ears. i can hear it, but it doesnt really bother me at all.

I never use AC, only the windows

Air trapped in the speaker compartment?

What is happening is that you are creating a low pressure area as air flows past the one open window. This sucks air out of your cabin until the pressure in the cabin is less than outside. At this point the outside air rushes back in (due to the lower pressure in the cabin). The pressure equalizes for a moment, then the wind "sucks" the air out of the cabin again. This will repeat until you open another window. Opening another window allows outside air to replace the air as it is sucked out of the open back window. Some cars will be worse than others because of the way the air flows past the open window.

my 93 ssei, does it even if both rear windows are completely down, with the sunroof and front windows closed. Also, my honda pilot sounds like a 15" sub with the lowest note going through it with just the rears down

Helmholtz resonance:
"When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time."

Consider it the springiness of the air inside the car being bounced high and low on pressure. If air weren't so spongy, this wouldn't happen.

http://en.wikipedia.org/wiki/Helmholtz_resonance

Well I guess that about answers the question then, it appears to be quite common and must be due to the design of the car, I have had plenty of other vehicles and this was a first for me.