system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why it pounds when you roll down a window in a fast moving car.

Why does it happen in some cars and others not? Also I have noticed it happens mostly in back windows, why is this?
gpt
There is a low pressure vacuum around your car due to the fast moving air. This vacuum naturally sucks the air out of the car but is not powerful enough to totally equalize the pressure. The popping is a result of the air inside the car quickly alternating between a high and low pressure state. 

Suck the air out of your cheeks and then open your mouth. It makes a nice pop. The cars windows are basically doing this real fast.