Balanced XLR, large-diaphragm condenser mic.

Agreed that it could be inductive noise from somewhere as opposed to vibrations, but there are times when I think I can faintly hear what I think is the same hum in the walls of the house with just my ears. Sometimes it stops suddenly and I only realize it stopped because it was suddenly gone.

But when I step outside I don't hear the hum. Or at least, there's possibly enough other environmental noise to cover it up.

Makes me think the vibration is coming through the ground, and that only the resonance of my 2-story house is making it audible.

It could be something in the house itself, but everything that vibrates (fridges, computers, hot water heater, furnace, etc) I've been able to rule out.