I have the same problem. I have 5 of these detectors that are interconnected through AC. They are almost 13 years old. Replacing the batteries on all 5 did no good. Removing the batteries altogether and turning off the circuit breaker also did not stop the chirping. I finally removed the three detectors from the upstairs, and to my surprise the chirping continued from the ceiling, not the units themselves. I removed the mounting base and all I see are the wires and the connector - I'm not even sure what the heck is chirping!
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RE: Chirping Detector
The 86RAC is an older model detector. The reason why it is chirping is because it needs to be replaced. Smoke Alarms only last 8-10 years and once the Americium located inside a detector is depleted, it will make that sound. The Replacement model is a BRK 9120B.