It waxes and wanes, but especially in the right kind of structures, the sound is a bit amplified by it seems, the nature of a roof --- it is sort of a drum head that resonates with the hum.
The hum is a lot like a railroad engine or river tow boat (uses the same engines by the way) heard through the ground from a very long distance.
For example, here in central Ohio, on a very calm night, I can hear the railroad engines at a freight yard, five miles away, when the engines are at idle.
Christmas day, bright and early, in 1993, I even detected a bad bearing in one of the engines. It was extremely cold outside, and nobody was on the road.
But I was determined to track down the sound coming from my bedroom wall.
I left the house and traveled west, occasionally stopping to get out of the car and give a listen.
Well, miles from here, I found a frieight train engine which a crew had left idling, and the engine had the bad bearing.
I left them a note on their windshield, regarding same.
Probably, the sound made its way here, because the whole area has an underlying limestone rock bed. The bed is lower, underground, near the freight yards, but it is exposed, here, where the soils are quite thin.
While I have not heard the humming in Taos, because I have not been up that way, I have heard the humming in the Santa Fe / Los Alamos area.
The caldera of which the mountain, Redondo, is prominent, making up part of a very large, and ancient volcano-appearing hole in the earth, and also next to which Los Alamos is built, might instead of being a "pure" volcano, actually be an ancient impact crater, where the crust was cracked well enough to evoke a volcanic reaction.
It much more looks like an impact crater --- very much like those on the moon.
Sometimes, it seems that whatever real impact that great meteorite had, the consequences still resound.