Wpe had the same issue. We have both hard wired with battery backup and solo battery devices. 10 in all. We had that single chirp every 30 minutes or so for months. We removed and replaced all the batteries to no avail. We checked each and every device however because it was a random chirp we could never pin it down. We finally disconnected all the devices and still heard the chirp. One of my daughters, with a fantastic set of ears, was staying the week end and asked me when I was going to change the battery in the "Carbon Monoxide Detector" . The beeping in the hall way kept her awake and she pin pointed it right away.