Have you had exposure, especially prolonged exposure to Flourecent Lights that hum ?
Maybe too long listening to a Leaf Blower ?
..And of course, any subwoofer or Bass Speakers ..especially in a high SPL enviornment, like the inside of a automobile.
I battle to keep away a hum too. I'm mostly successful. And those 3 things listed above are what i have in my life ( to avoid like the Plague ) to not have a hum later on.....
Recently, had the hum, AND SURE ENOUGH, i remembered i had been around some humming ( noisey ) Flourecent Lighting.
Hope you figure out what it was. Your ears obviously don't like it ( though you may want to disagree with them ) Ears don't lie...... ( so to speak....