If you shut the power off and it still hums, then maybe it's something
that has a battery ? I agree with the stethescope idea, narrow it down
as much as you can and cut a small hole in the wall. Repairing drywall
is easy and cheap (though you may not have matching paint).

Do you have a radon blower ?

wrote:
HELP! I'm at the end of my string!! We have been struggling with a
sound that we hear in our wall. It just happens to be the bedroom
wall...so it makes sleep difficult at times. We are hearing a humming
sound...it honestly sounds electrical. Originally, I thought I'd
chased the sound to the outside power meter. We had the Power Company
come out...TWICE...they checked the meter and all was well with that.
We began shutting off power to the house room by room....still heard
the sound.....we just shut the whole breaker off...STILL WE HEARD THE
SOUND! It's been going on for weeks and I'm super concerned that it
could cause a fire. I'm not sure what to tell an electrican if I call
one...gee, there's this sound that comes from my wall...sometimes it
does it and sometimes it doesn't. ??? I have been reading online that
it could be water pipes....or from the hot water heater...the hot water
heater is not far from this wall, but it's an exterior wall...so I
don't think there is pipes running in it....When I put my ear up to the
wall, it sounds like a toaster oven's hum...or almost like something is
shorting out....We've even shut the power an tightened up any loose
connections from the breaker box. HELP!!! SUGGESTIONS?! We are first
time homeowners (with a new baby and short on cash) and are at a loss
as to where to go from here! My personal email is .
I joined this group to ask this specific question, but I'm not sure
that I know how this group thing works...maybe I can figure it out
better than I can figure out the humming wall sound! (no bees or the
like, no doorbells...all those have been checked!)