60Hz audio has a wavelength of about 5m. In theory you should be able to use two microphones and locate the source by the phase shift, if you've got an oscilloscope. (Something like: start with the two mics close to each other, check that the hums are in phase, and try moving one mic so that they stay in phase. If you find yourself moving the mic in an arc, the centre of that circle is the source of the hum. If you find yourself moving it in a straight line, the source is distant and its direction is perpendicular to the line. If the hum is in phase everywhere in the room, you're in the resonator situation.)

Peter