Have a rural house, low level humming (sounds like a generator on 24/7) is constant with no break in level of hum. My neighbor and I have gone to great lengths to find the source, he has decided it is the monitoring pumps at the county landfill which run constantly and their noise follows the river and flows into our valley. It is over 8 miles from here, so I question it, but have searched for the relay boxes on our phone lines to see if there is one that is generating that sound. Our area has frequent disruption of phone service with storms and road crews are always working somewhere. I have been near major power lines and while I note the noise when I'm near them, they do not follow me to my home (out of state) and they are less than 2 miles away. What I don't understand is why when I plug my ears, all noise is blocked except this particular humming. While searching for an answer to this problem, a suggestion online was to contact your county/state environmental agency for a noise assessment/abatement solution. They may actually know what is generating the sound in your area.