There are a couple of things to be aware of with the smoke / carbon monoxide alarm.
No one can tell where the chirp is coming from. I have been on several chirping sound service calls where the homeowner wasn't listening to the right device. The homeowner though the chirping was coming from the smoke alarm on the ceiling, or from the fire alarm horn on the wall. The chirping sound was actually coming from a plug-in carbon monoxide alarm near the floor.
You probably have a good idea as to which alarm is chirping. However, even for professionals like me, the only way of telling for sure which device is chirping is to get very close and listen to a chirp. If the chirp is really loud when your ear is closer than half an arm's length away, then you have found the right alarm.
Only after checking it out that way should you go to the troubleshooting part.
All carbon monoxide (CO) alarms have a shelf life. The shelf life may be 6 years, 8 years, or 10 years. This is not the time you have from the time you install the CO alarm; this is the time you have from the date of manufacture to the end of its usefulness.
At the end of the useful shelf life of the CO (carbon monoxide) alarm, the alarm will start to chirp. When it starts to chirp, it will continue to chirp until it has been replaced. There is no fixing a CO alarm after its shelf life has expired.
Sometimes, the detector in the CO alarm fails before the end of the official shelf life. It will chirp even then.
Replace the Alarm
After confirming which alarm is chirping, replace it. Replacing any other smoke alarm or smoke / carbon monoxide detector of the same vintage might be a good idea, especially the carbon monoxide alarms. If one has exceeded its shelf life, the others may not be far behind.