When you hear your carbon monoxide alarm beeping, the first thing you should do is pay attention to the exact nature of the sound. Different patterns of beeps or chirps mean different things, though all signal that you must take some action.
A single chirp every 30 or 60 seconds means the battery is low and must be changed.
Chirping that continues after the battery has been replaced means the alarm is at the end of its life and the unit must be replaced.
A loud steady alarm (not beeps or chirps) = EMERGENCY. The unit has detected carbon monoxide gas in your home. Get everyone to fresh air and phone 911.
Learn more about carbon monoxide safety