[Vaguely house related; no place else to put this]

While trying to fall asleep last night, starting at about 11:15PM I
heard an obviously electronic "BE-BOOP" sound repeated at intervals of 2
to 3 minutes. It went on for a few hours, keeping me awake as I went
slightly nuts trying to figure out what it was.

The sound was similar to the sound made by police and commercial two-way
radios to let the parties know that the sending station had stopped
talking. There's nothing in the house that has ever made that sort
of sound, and nothing that is supposed to make that sort of sound.

It was clearly not coming from inside the house, and was only audible
from the bedroom and kitchen which have windows which open to the back
yard. There are neighbors on either side and an more across the alley.
Our back yard has a 5' cedar fence all the way around, and the two rear
gates are locked.

So far as I could tell, there were no people, cars or trucks in the
alley, and there was no sign of anyone doing anything in the nearby
yards.

I didn't go outside and look up to see if there was a UFO hovering
overhead, so that possibility shouldn't be excluded.

Anyway, would anybody care to offer some insight or guesses about what
sort of thing I might have been hearing last night?

--
St. Paul, MN