Good luck. This does remind me of a noise I used to hear at nights when I lived out in the woods. It seemed more noticeable in winter and at night. I finally decided it probably came from some kind of farm machinery at a goat farm a couple of miles away and across the river (like something using a compressor or some kind of prosessing) or one of the few remaining mills (but I couldn't see them going all night anymore)---because it was consistent, I got used to it & after a while didn't even notice it. Cops generally are not good dealing with these non-emergency (well, I might hesitate to call in an emergency depending on what that emergency was...) you-have-to-actually-listen for a few minutes and problem solve.
I found the noise I heard difficult to place in more than a general direction & it did sound like it came from out of doors or inside a non-insulated thin-walled structure. I decided the season made a difference because of the loss of forest leaves etc.