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Saturday morning about 4 a.m. I heard this chirping noise: about one chirp per minute. The only time I'd heard anything similar was from my smoke alarm. I tried vacuuming the the dust out of the thing. I even disconnected it. Still the horrible chirping. Could it be the carbon-monoxide detector that I'd bought last year in response to a new Mass. law? I unplugged it. Still the slow, maddening chirping continued. I went to the Firex web site. Replace the battery when warned by chirping, it said. O.K. Since it was hard-wired I'd assumed there was no battery, but maybe there was. I took the thing down, but couldn't get it open. I took it to a hardware store. The clerk couldn't get it open. Then an elderly hardware employee looked at it and said, this is the hard-wired type of smoke detector. It doesn't have a battery. Could it be your carbon-monoxide detector? No, I unplugged that. I went back home and called the local electrician, who was able to come about 3 hours later. I told him my story and said it must be the heat detector. But no, it was the carbon-monoxide detector that does chirp, even when unplugged, because it uses the waning power of the battery to produce the chirps.