For well over a year, we've had a baffling problem with chirping smoke detector(s) (hard-wired with battery backup) in the upstairs hallway of our 7-year-old, 4-bedroom colonial. One or both will chirp a total of 4 times, about 17 seconds apart. Replacing the batteries on both didn't solve the problem. I disconnected all 6 upstairs detectors, removed the batteries, "drained" the units, put in new batteries and re-connected them to the wiring connector. That didn't solve it.

We tested the system, and all 6 sounded, plus the one in the basement and the one in the main-floor hallway worked, too. But the chirping continued. Frustrated, I ended up replacing every unit (all are Firex units with 9V battery backup) and even tested the power in each battery, replacing where necessary.

Still, the chirping continued. Any time of day. Sometimes, the chirping seemingly alternates between the 2 locations in the upstairs hallway. Finally, I ended up pulling out all 6 upstairs units (but put the batteries in the bedroom units and set them on dressers to still provide some safety protection). Yet the chirping still happens in the hallway even though there are no detectors there at all! Seems like it comes from the same location in the hallway ceiling. I went into the attic, couldn't find 1 in the area where I hoped one might be, without risking falling thru the ceiling and disturbing too much insulation by trying to get to the area above the other end of the hallway.

When we test the system by pressing the buttons on the still-connected main floor or basement units, there is no beeping in the upstairs hallway, so I'm figuring IF there are 2 units in the attic somewhere, they're maybe not wired with the others. When I hear chirping, I have quickly climbed into the attic, I can hardly hear anything, perhaps either unit is muffled by the rolled and blown insulation.

Looking for these units, IF they're up there, might be like looking for a needle in a haystack under all that insulation. Should I just continue to let them chirp several times a day (here they chirp again right now!)until the batteries are dead? I don't want to begin cutting ceiling drywall.