system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening when you get helicopter car? (Popping noise when you open car window)
gpt
The air in your car is resonating. You know how when you blow across a bottle you get a specific tone? The same sort of physics is happening, but now the bottle is your car. The analogy actually generalizes pretty well - a more full bottle has a higher pitch, and a more empty bottle has a lower pitch (i.e. coke bottle vs milk jug). The car is like a giant bottle, so the pitch is really really low, which makes that miserable popping. Since it's a sound wave, you're really just experiencing wide swings in pressure, which is why you can really feel it in your ears. 

There's an easy way to get rid of it too - crack a window on the other side. It's like drilling a hole in the bottom of your whistling bottle. If you've got a hole on the other side of your car the pressure can equalize, and you can't set up that resonance.