I've walked around the room with the mic in my hand and the headphones on. The sound changes as I move around the room, but not enough to help me narrow its location down exactly . It seems to be coming from all the walls and floor. As if the whole house's frame is quietly resonating to a distant low frequency Diesel engine of some kind. That's what it sounds like anyway.
I'm relatively sure it's not hum in the gear itself, otherwise the noise would be the same as I moved the mic around. Also I've been through several different bits and brands of gear and the noise never changes.
I have had situations where the electrical noise was coming from wiring or equipment below the floor or overhead, and as the microphone moved around the microphone cable followed (naturally) and the induced noise changed as the cable moved. Transformers used for lighting can sometimes leak a noisy magnetic field. As can a nearby outdoor utility transformer feeding your neighborhood or commercial facility.
Good (balanced) microphone cables are supposed to reject outside magnetic/electrical interference, but this may not be 100% effective. Cable shielding can also be helpful but can be overcome if the cable is not perfect or the noise source is relentless.
Were you testing with a wired microphone? Balanced audio connection?
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