First off, can't wait for a cookie.
Second, are you sure it's not the sound of your (back up) battery dying? Most of them are programmed to have annoying beeps to let you know it's getting low so you can change the battery.
I'm sure if it was Carbon Monoxide & there's an alarm for that you would def know it, as it'd probably be beeping you into insanity. They're really sensitive & can usually detect it before it's at a dangerous level.
Just my 0.02, if it helps, good, if not at least I helped you kill about 1 whole minute reading it.
