After two days, it's gone from bugging me to driving me insane. There is an emergency exit sign or fire alarm with battery back-up in the apartment hallway, apparently the battery is dying because it is periodically chirping to draw attention to its battery problem. I have called the apartment office, but nothing has happened to fix it, this is the fourth day, with the chirps becoming more frequent. And because they are designed to alert you in an emergency, they are a particularly piercing chirp.