I heard it for years....turned out to be a train. It was so far away that you couldn’t hear the engine or horns.
The humming sound and rumble transmitted through the ground. As it went up a long hill, it would shift up and down constantly causing two tones of slightly different frequency.
Finally one day I was on the other side of town and could hear “the hum”. I asked the guy I was visiting if he could hear that. He told me what it was.
It’s called infrasonic noise or sound. It is the effect from ground waves or vibrations, not air waves, and it is low frequency which may account for the hum or whatever. You feel it as much as you hear it.
It does sound like a train engine. Like when it’s off in the distance. We had trains around where I grew up. It sounds just like it. You couldn’t see the train, but you knew one was coming by this sound. The sound may be bouncing off the mountain sides.