Beeping from attic

We recently moved into a used house and a few months ago noticed a high
pitched beeping sound which seemed to originate from a particular
location of the ceiling in the hallway. This beeping sound continued
for several hours, until finally it started to gradually become quieter
and quieter, until after an additional hour or two the sound faded
away... as if a battery had been depleted.

We haven't yet entered the attic, as the barely human-sized access hole
in our closet isn't very inviting.

Can anyone offer any theories as to what the source of sound could have
been? ...how about advice on climbing into the attic?

Please copy your response to if possible.

Thank you!

Chris