Chirping from a hard-wired fire alarm
Even the wired smoke alarms have a end of life date.  They start to chirp when they are not functioning up to spec any more.  This can be caused by accumulation of particles on the sensor (from smoke, fumes, etc.)  When one of mine starting chirping (single beep every few minutes or so), I took it out and noticed that the install date was about 8 or 9 years ago, for a product that was supposed to last 10 years.

You can probably remove it from the base plate and look at the back side.  There should be a manufacture date on it and instructions regarding how long the device should last.  If it is anywhere near that date plus the expected product life, then replace it.  

I would replace all of the smoke alarms in the house at the same time if they have similar dates on them.  These things start chirping at most inopportune times.  (Seems like they know when it is most annoying).  The safety of your family is worth it, and knowing that they are all good is peace of mind.