“chirping” that occurs when a battery-powered smoke detector begins to
go dead, and your entire family will be alerted in the event that a fire
does occur since hardwire smoke detectors can be interconnected.
Off-topic, but the sensors in these hardwired units expire after 10 years, and they start beeping. The batteries in modern battery-powered units with wireless links expire after 10 years, along with the rest of the unit, and they start beeping.
There are exceptions, the first-generation Nest was pretty bad.