That's likely the sound of jetwash striking (nearly) stationary air as it exits the engine.
I'm familiar with the roar of jets taking off. Some years back I happened to be biking past an airport, at the end of the runway, just as a passenger jet was lining up for takeoff, headed away from me. I thought I'd pause to watch.
Only as the engines spooled up did I think, "hrm, this could get loud".
It didn't.
Instead, what I heard was ... the engines spooling up. Loud, yes, but not a roar, just an increasing pitch, until the airplane started accelerating down the runway.
It wasn't until some 15-20 seconds later that I heard the familiar roar, echoing off of hills five or so miles away. That's when I realised that the whine was the sound of the turbines, but the roar was the sound of exhaust gas, streaming out of the engines, hitting stationary air and generating intense turbulence, and radiating outward in a perpendicular line to that jetwash. So I didn't hear it directly (it was moving away from me), only the reflection (as that wall of noise, now reflected off the hills, was directed back toward me.
I doubt the vortex would make any particularly loud sound, though you might hear the rushing of air. Speeds are in the tens of miles per hour rather than hundreds as with jetwash.