OK, so I'm hearing it too....and I don't believe in any explanation but scientific fact....There HAS to be a logical explanation here, especially assuming everyone here is hearing the same sound.
For me too, in my home, it is pretty much constant. I am not yet prepared, however, to accept that it is caused by "resonance from beneath the earth" or any "taos hum" theory. No, I still maintain it's got to be something to do with airflow from somewhere or maybe an electrical issue.
I've had the plumbing checked. Done the electricity and plumbing tests by turning everything off etc..I've gone into the attic to see if I can hear it from the vents but it's no louder there than anywhere else. I can hear it faintly in my detached garage. One night, I went next door and could hear the same sound in my neighbour's house. (The wife there could hear it too but the husband couldn't). A couple of times, I've been able to pick it up while standing in the middle of the street. Mostly, though, you can only hear it inside.
It doesn't have a rhythm. It's not a buzz. You can't even locate it by listening to the walls. It's more like a wheezing hum, kind of "up, down, down up, up, down...."etc as others have said, like a car idling in the distance. There is more than one note to it which could indicate more than one location or source. As I understand it, any noise can travel through studs and plumbing in a house. It's got to be something to do with that, I think. Called the electricity provider. They were no help at all - even though I have one of those green transformer boxes on my front lawn. The man I spoke with said that if I can't hear it when I listen to the box, it ain't coming from the box, period. There are definitely times, when one can hardly hear it, though, which makes me wonder....wind?
My next approach is to clean out the bathroom fan vents and the dryer vent - all on the second storey of my home. I read somewhere that airflow can resonate through these. I have my doubts though and once that has been done, I'm gonna' be stumped for good!