Maybe you've previously read of the Taos hum. It's named for the town it's heard in — Taos, New Mexico— and it's been driving people nuts since the early 1990s. Just a fraction of the population hear the hum: about 2 percent.
But other hums pop up across the globe, and the 2 percent figure is pretty across-the-board. In 2011, the town of Woodland in England reported the presence of their hum, and they joined a list of towns that includes the English town of Bristol, the Scottish Largs, the Australian beachfront town of Bondi, and, of course, Taos. Most people who hear the hum (who are rather unimaginatively called "hearers") are between the ages of 50 and 70, and the noise has been linked to headaches, nosebleeds, and insomnia. At least one person was even reportedly driven to suicide because of the never-ending, maddening noise, which is something like the low throbbing of a diesel engine running constantly in the background of your mind.
One hum, heard in Kokomo, Indiana, was explained away as coming from a compressor and fan on a nearby industrial site. The others remain unexplained, though, and theories range from farm machinery to UFOs. The smart money, of course, is on the UFOs.