Noises that never stop are the worst.
I have pretty bad tinnitus. From years as a drummer/musician, my hearing is kind of awful, and the ringing never really stops. At night, I have to sleep with a white noise maker because otherwise the ringing will keep me awake. It's nerve-wracking to say the least. Even borderline maddening at times.
When I lived in Bangor, the rooftop climate control unit on the top of the small plaza on Broadway next to Tractor Supply, would often cause this unending hum in my house. It only stopped if I opened the windows. It was weird. All I could figure was that the low end somehow reacted with my house and made everything hum.
Some Hampden residents are complaining about their own unending hum.
On the Living In Hampden Facebook page, a few residents were commiserating that there was a hum that just never stopped. They were saying they could hear it outside, inside... all over. So folks began to theorize where it might be coming from. Most folks seem to think it's likely the PERC Plant across the river in Orrington.
Seems a likely candidate. I've also heard people talk about the wastewater treatment facility on the border of Bangor. People have said they feel like they can hear plenty coming from there. I can't say I've personally ever heard it. I've certainly smelled it from time to time.
It's hard to say what the true culprit is. I know my heat pump makes a lot of weird indoor/outdoor noises. As does my radon mitigation fan. I'm not saying that's what these folks are hearing, but it's a couple other possibilities. No matter what it is, I can sympathize. It's maddening. Hopefully, someday we get to the bottom of it.