Problem for me isn't "what's that beeping". It's "Which one is beeping" at a frequency that I can hear well but can't tell direction. It's a similar frequency to the one they use for hearing tests. I could tell they if I heard it but not which ear.
Keep in mind that if your smoke detectors are powered primarily on AC with 9V back-up, they are probably wired together. One "beeps" and they all "beep", just not at the same time. If one battery is on the "BEEP", chances are they all will be in the next few days. Replace them all so you only chase them once.
Don't take the battery out and forget it cause it stopped "beeping" either. I am certain at least one of the fire/rescue guys on this board has had to carry the body bag of the person who removed the battery from the "smokie" and forgot to replace it.
UTAlumnus how well did you enjoy your time in AUSTIN during college?? Ha Ha