Why, when one opens 1 car window, does that noise occur?

The car is behaving like a closed pipe, so you get a resonance set up. There's a Wikipedia article here, but for once the Wikipedia article isn't that great, so there's another better article here. I imagine you (like most of us) will at some point have discovered you can make a sound by blowing across the top of an opened bottle, and it's the same thing happening in your car with the open window acting like the opening in the bottle. Since your car is much bigger than a bottle the resonance frequency is uncomfortably low.

When you open a second window you get an air current flowing through the car and this destroys the resonance.

In another post of a supposedly identical question today, which was closed while I was writing the following answer, it was claimed that the frequency did not change if the opening of the window was changed. And Helmholtz resonators change frequency with the area of the opening. Therefore I post my answer here:

The excitation of air oscillations in a moving car interior due to the open window is probably due to a vortex shedding phenomenon at the edge of the open window which creates vortices and thus air oscillations with a frequency proportional to the speed of the car. This phenomenon causes the whining or howling that is emitted from objects (e.g. ropes) in strong wind. These oscillation can also couple to the resonances of standing air waves in the car interior which would enhance the effect.