As always, a preamble. There are many hums. I refer here to the classic Hum, described in the research literature and reported by Hum sufferers at http://www.thehum.info.
I suspect that the Hum is a biological reaction to the multimode propagation and subsequent interference of VLF electromagnetic energy, compounded in some cases by existing sources of otherwise inaudible low frequency sound and infrasound. It is an activation of the auditory system detectable by a small proportion (less than 5 percent) of the population who are acutely sensitive to the presence of low frequency sounds or who have specific anatomical conditions. Increasing numbers of increasingly powerful VLF transmitters, via ground wave, skywave, and magnetic conjugate propagation modes, create ground interference and standing waves that create locations with intense levels of VLF energy. The odd behaviour of the Hum is caused by diurnal, seasonal, and geomagnetic disturbances affecting the ionosphere.