Used to hear something like that from 757s.. Soon after they passed overhead around 200ft on final approach a sound like a whiplash would be heard as the vortex hit the ground. Strangely, I never noticed it with heavy jets. I'm quite interested that the effect could be heard when the aircraft was at 500m (1600 ft..ish) as I am not aware that ATC provides wake turbulence procedures to aircraft which are vertically separated..