I can offer my humble two cents, FWIW

I get that whenever I've had contact with something that rumbles, vibrates, runs, or otherwise moves at a fast rate. Examples would be holding my hairdryer with the wrong hand (the motor fan moves fast), leaning against my fridge or freezer when they're running (again the motor), holding the weedeater with the wrong hand (the little whip thing goes around), or even leaning against a car when the engine's running.

You say that you were on a riding mower before the buzzing started. Well, I dunno about you, but that would sure "do it" for me.

Retreating back under rock,
Hladna.