What exactly is/was the Taos hum? Has it stopped or does it still exist and continue to produce the same noise?
ULF waves from a source close to the area of Taos. Another one is near Detroit, MI, hence the ‘hearers’ experiencing it in that state.
Sorry, Ultra Low Frequency waves are used to communicate with submarines while deep below the surface of the ocean.
This is the most sound theory I have hear, pun intended. 
I’ve been in Taos many times and the only thing I heard humming was a bum and a bookstore lady with a cat.
Taos dwellers are those too wacky for even Santa Fe
[From the Castle Anthrax]
… and then the New Mexico oral sex!
[/From the Castle Anthrax]
Hasn’t stopped.
So far all sensible theories came to naught. It was entirely subjective and WASN’T low frequency sound or EM waves. Researchers at the U of NM went looking for it and found nothing, yet some of the scientists themselves could hear the “sound.” By comparing the perceived sound against an audio oscillator, its frequency was found to be around 75Hz which is not at all subsonic. The UNM performed hearing tests and found that the few percent of people who could hear the hum did NOT have unusually sensitive low-frequency hearing. One telling point: the sound continued as normal when there was a wide-area power blackout in the Taos region, so it probably wasn’t caused by anything manmade, or at least anything local.
On the other hand, people claim to hear similar “hums” in many other regions, and some of these were obviously industrial noise (easily detectable with a good microphone.)