Cool ideas, but we're on a city sewer and I'm almost certain that we don't have a booster pump. I know that would have come up. A little after when we moved in we had water damage and had to gut the master bath. I'm 99% certain that all we have is the main water heater (which we also had to replace this winter - don't get me started on the things we've had go wrong with this house).

The toilet area does not have a window, but the bathroom does. However, by the time I get anywhere near the window I can't hear the noise anymore. I really can't hear it very well at all if I'm outside that toilet closet. Even when I'm in there it sounds like it's very far away.

I should mention that I've been up in the attic, and I've had workers up in the attic, and none of us have ever heard anything up there.

*edit*
And the only thing that's below the toilet is the garage. I can't hear anything in there either. That's one of the things that makes this so weird: the way this noise sounds in that toilet closet, you would think that it would get louder if you moved in some direction. I can get to almost every single side of that room except the area over the garage and part of the attic. I'll have to figure out how to get in those areas (the garage one is particularly difficult).

I'll try turning the power off first...