I've read a lot about this in the last year. Some hum reports are traceable to localized sources (turbines, pumping systems, etc.). Then there's The Hum, proper, which has been reported since the 1970s in Britain, and the States and elsewhere since about the early 1990s. As one researcher put it: if you find a source, it's not The Hum. Tinnitus is usually ruled out, except by some of the more dismissive Hum investigators, for two reasons. First, tinnitus-type sensations (usually high-pitched ringing, sometimes rumbles/flutters) don't generally correspond to the sounds described by Hum sufferers ('Hummers'). Some say it's like a steady, low tone; a lot of others compare it to trucks idling in the distance - i.e. low, roughly modulated. Second, an internal source wouldn't explain The Hum's geographic distribution (often in clusters, or straight lines - hence the suggestion that water or gas backbones might be culprits).
There's a lot of debate over whether it's an acoustic or electromagnetic phenomenon. It's most often perceived indoors, suggesting that vibratory energy is resonating strucutres of the right shape and size. But, it gets sticky. It starts to follow people wherever they go. So that raises more questions. Is it partly a matter of anomalous sensitivity to something that most people don't notice? And what roles do memory, anxiety and anticipation play? Some former Hummers have said they simply got tired of worrying about it and it subsided. The Hum rarerly shows up in acoustic measurements but most researchers consider that a failure of technology or methodology rather than eveidence of mass delusion.
But an acoustic explanation wouldn't explain its global spread. So that leads some researchers to argue that it's actually an electromagnetic effect. We're not supposed to be able to hear electromagnetic waves but the suggestion is that a small portion of the population percieves them in a sound like way. The electromagnetic argument tends to lay the blame on military-industrial megaprojects like HAARP and TACAMO because they're the only things that could blanket the earth in the sort of patterns associated with Hum distribution.
But, connect_icut, if you're both hearing (or half-hearing) it now, maybe keep an eye out for potential sources of LF vibration that could be finding a resonance in your place. Ventillation fans, etc. (e.g. elsewhere in an apartment building) can make a room do funny things. And from what I've read (and my own experience being peturbed by neighbour noise + watching local NIMBYs) the more you can ignore it, the less it will bother you. The worst thing is to be constantly scanning for it, imagining it when it's not there, etc.
