Where is the chrip oe beep comming from ?

My security system has been disconnected including my smoke detectors.BUT I still have a beeping or chriping sound comming out of the ceiling.When I go into the Attic I can not hear the Beeping. Is it posible what ever is making the beep SOUND LOCATED IN MY HEATING/AC DUCKWORK ? Do they have some type of security ALARM hooked up to the AC/HEATING unit in the ATTIC. This beeping about every moment is driving my dog crazy. Please Help Me.
Thanks,
Bernie