In this heat wave, driving with an open car window has brought back that hammering noise you get when driving along. We thought it might be like the way water gulps when being poured from a bottle and air has to force its way in to fill the space. But apparently it is the same as blowing over the neck of a bottle that is part full of water to get a note. This is called Helmholtz resonance.
https://jalopnik.com/why-do-slightly-opened-car-windows-make-that-awful-soun-1447498738
