Smoke alarm beeping.
My sublandlord (a real appliance repair person) with a ladder.
Me: Here, you want a battery? Take mine.
Him: No, let me see what it needs first.
I guess there's one possibility if there's a whole bunch of lint or crap on it. I don't know if that causes the beeping though.
Because how my house is laid out, it is hardwired and networked so the battery is strictly a backup in case power goes out, so if one alarm false triggers or has a dead battery, I hear beeping all over my dumpy house.
Amazingly, those networked smoke and fire detectors are cheap. I got the contractors grade in a plain brown box for $7 each.