[eye diagram]

If I open either (or both) rear windows at normal highway speeds, there is a loud vibration/oscillation (about 5-10 times per second) of the air inside the cabin. It's similar in nature to 'water hammer' in plumbing, but never stops. It's so loud it hurts my ears.
Opening a front window reduces the vibration. I figure it's some sort of aerodynamic issue. For the technically inclined, I would describe this as a relaxation oscillation.
Has anybody else experienced this? It's not subtle.

 

[shocker]

Commonplace on modern vehicles, it's a side effect of low drag aerodynamics. I know for the Volt there were/are kits that modify the side mirror housings to reduce the "thrum".

Presumption on the automakers part these days is nobody drives with the windows down. I don't, it's too hot, too cold, raining, or there's too much pollen in the air.

 

[PhilBo]

What you are experiencing is normal. A front and back window needs to be open in order to eliminate the vibration. Experiment until you find the best opening for both.

 

[XJ12]

p. 52 of my 2017 owners manual:

The vehicle aerodynamics are designed to improve electric range performance. This may result in a pulsing sound when either rear window is down and the front windows are up. To reduce the sound, open a front window.

 

[Enzeder]

These articles can say it better than I:
https://jalopnik.com/why-do-slightly-opened-car-windows-make-that-awful-soun-1447498738
https://www.caranddriver.com/news/a15344069/explained-that-weird-throbbing-when-you-open-one-car-window/

We had a car that had a setting for the sunroof that would open it about 75% because if you opened it 100% with the windows up you would get that resonance.

PS: Cracking a front window a few inches usually helps a lot.

 

[GregBrew]

If I open either (or both) rear windows at normal highway speeds, there is a loud vibration/oscillation (about 5-10 times per second) of the air inside the cabin. It's similar in nature to 'water hammer' in plumbing, but never stops. It's so loud it hurts my ears.
Opening a front window reduces the vibration. I figure it's some sort of aerodynamic issue. For the technically inclined, I would describe this as a relaxation oscillation.
Has anybody else experienced this? It's not subtle.

Every car I've ever driven does this. I used it as an "independent experiment" for my high-school physics classes. Then I'd have them write up an explanation for why cars do this, and relate/explain the frequency of oscillation to the sounds made by both a flute and piccolo. (Flutes and piccolos operate by similar principles.)

I "fixed" the problem by opening the front passenger's side window and rear driver's side window each about 4cm. It allows air to flow through, without oscillation (or mussing up the hair).

 

[NY-Rob]

If you open a front and rear window an equal amount that's usually sufficient to prevent or stop the oscillations. Unequal openings between front and rear windows will start/increase oscillations.