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.
D-2274 Volt - gone but not forgotten
188005 2017 Bolt Premier
"This movie (Dr. Zhivago) pulls so many punches and it STILL makes Communism look like the worst idea any human being has ever had"