I have heard this hum. It has been a while, but I use to live in Camp Verde Az. back in the 80's and 90's. It is a very small place, no factories, or anything like that. I would hear it at night mostly. A low machine like sound, I would say to my Husband " Can You Hear That "? and he would say " Hear What "? I would say " It sound like a diesel idling ". But when I would go outside to try to pin point where it was coming from it would be gone. I THOUGHT I WAS CRAZY! Thank You for proving that I was Not!
Forgot to add that when I would go back inside and lay down in bed it would be back. It wasn't every day in the beginning but 3 or 4 days a week , then it became so common I started hearing it daly/nightly. I now live in Michigan and have not heard it since. There is more going on here than simply to much caffine or an inner ear disorder. Some maybe but now all.

