Happens, it's not quite a hum, more like a dog whistle. Piercing. Not like the Black Mirror, constant or vibrating. But like a static discharge, or a dog whistle, whir or hum, becoming high pitch, and it really hits your inner ear. Everything else still sounds normal. My dog heard it once or twice, and it was really bothering for both of us.

No it wasn't a car alarm or a siren. They could still be heard if they are there, the same with other sounds in the background, on what seems to be an electrical frequency oscillating.

Don't know WTF it is? But has happened a few times in memory, maybe yearly, or every few years, and it was really bothering, because I didn't know what it was, or whether it was just me hearing it. But that pitch gets more intense until it stops, sometimes it has lasted a few minutes. I know the dog heard it, it whimpered and pawed its head. I looked for the source, and it was gone. No, it wasn't a prank, somebody else, and the other electronics, don't do it otherwise. Unless it's the TV, I have always suspected it. There is other stuff that beeps, freaking fire alarms can be annoying, just beeping once or twice for no reason, or the fridge and its hums. But not like that. Don't know what the hell it is.