Tracing the 'Hum' 6/5-11/2008 "Hum is NOT a noise in the normal (auditory) sense of the word; it is a perceived sound generated inside the head of the sufferer" per  & has had typically undocumented causes.  Suspected causes are both increased military [extremely low frequency (ELF) waves] and commercial [ionospheric high-frequency active auroral research program (HAARP) waves] environmental sound pollution. (Additionally, do not rule out the increased wireless communications used internationally.  Other possible causes are tectonic plate movements, "electromagnetism caused by meteorites, & electromagnetic waves produced by the interaction of the planet's magnetic field with solar flares."  Additionally, there is plenty of pollution from the outdoor hard wires, especially from the transmitters.) Other adjectives by which Hum is cited are Taos, Kokomo, Auckland, places where there have been case studies/reports of Hum sounds/symptoms.