The exercise everyone gives you first: record sixty seconds of the room doing nothing. Then put on headphones and play the nothing back. My kitchen's nothing turned out to be a chord — refrigerator at a 60 Hz fundamental with two quiet hums above it, the hood fan's hiss like weather, and once every nineteen seconds the furnace's boiler ticking like a cooling engine.
I'd lived in that kitchen for six years and heard none of it. What's striking isn't that the noise was there. It's that my brain had been editing it out in real time, all this time, and the recorder is the only honest editor I've ever hired.
Practical bit, for the one other person who cares: record room tone with the lights off (dimmers sing), and don't stand where you think the "center" of the room is — the floor joists have a sweet spot, mine is a half step left of the stove, where the refrigerator hum drops almost a full dB.