February 28 2016, 19:13

In Washington, near the Reagan airport, there is a recreational area where planes dump leftover fuel… Oops, I’m saying something wrong – it’s a place where locals relax – playing badminton, walking their children, and so forth. And above them, literally over their heads, a plane flies by every minute.

After a plane flies over, for several more seconds a distinct sound emanates from nowhere above their heads, generated by the air vortex lifted by the aircraft. It’s quite unusual when the sound comes from a place where there is nothing. Sometimes the vortex is visible, then it resembles a small tornado, but this requires dusty weather.

http://beinginamerica.com/2016/02/29/gravelly-point-park/