I am having the same problem. My experience is almost identical to what you described. It’s driving me nuts.
Did you find a solution?
Hello, I was googling this problem and came upon this thread. I have the same problem in my house. Started last fall, didn't notice it much over the winter but in the past weeks I've noticed it again. And like you guys I don't hear it anywhere but inside the house. Sometimes i can ignore it and sometimes it makes me crazy. I live alone so no one to ask if they hear it too.
Did you find a solution to the noise?
I know this is an old thread, but I wanted to add to the list in hopes of finding a solution/cause.
Currently, Turning OFF the water coming into the house makes the low rumbling/humming sound--which is under my raised house--go away.
In my situation:
I'll keep diagnosing and update with a solution should I find one!