The Hum phenomenon

Many, if not all, people have heard of the Hum, a worldwide phenomenon involving a persistent low-frequency humming, rumbling, or droning noise. Reports have come from the United Kingdom, Australia, New Zealand, Canada, the United States, Japan, and Germany. It's very difficult to detect with microphones and its source and nature are hard, if not impossible, to locate.

The essential element that defines the Hum is often described as being comparable to that of a distant diesel engine idling, or to some similar low-pitched sound for which obvious sources (e.g., household appliances, traffic noise, etc.) have been ruled out. Many people hear the Hum only inside buildings as compared with outdoors. Many also perceive vibrations felt through the body. Earplugs are reported as ineffective.

In Britain, the most famous example was the Bristol hum that made headlines in the late 1970s. During the 1990s the Hum began to be reported in North America and to be known to the American public, when a study by the University of New Mexico and the complaints from many citizens living near the town of Taos, New Mexico, caught the attention of the media.

June 9, 2011, it was reported that residents of the village of Woodland, England were experiencing a hum that had already lasted for over two months.
It's also been reported since 2010 throughout Windsor and Essex County in Ontario, Canada, where some residents claim it to be correlated with the time of day, or week, while others seem unaffected or unable to hear it.

April 20, 2012 the Canadian Government decided to officially investigate, and the launch of a study was announced on January 21, 2013. Current suspicions are that the noise originates on Zug Island near the southern city limits of Detroit in Michigan.

The Hum has also been heard since at least 2004 by residents on Canada's southwest Coast in the region around the city of Vancouver.

The Hum has also frustrated residents in County Kerry, Ireland. The phenomenon was also recorded in 2012 in Seattle, and Wellington, New Zealand.

The World Hum Database and Mapping Project was launched in December, 2012, in order to build detailed mappings of hum locations and to provide a database of Hum-related data for professional and independent researchers.

Some explanations of hums for which no definitive source has been found have been put forth.
Tinnitus has been suggested by some physicians. It's generated internally by the auditory and nervous systems. However, this theory fails to explain why the Hum can be heard only at certain geographical locations. People who both suffer from tinnitus and hear the Hum describe them as different, and many can find locations where they don't hear the hum at all.

Spontaneous otoacoustic emissions, where human ears generate their own noises. People that hear these sounds typically hear a faint buzzing or ringing, especially if they're otherwise in complete silence, but most people don't notice them at all. However, these emissions occur with equal frequency across age groups within the population, and the Hum typically occurs in regional clusters, and rarely within large metropolitan areas.

Researchers have also tracked down a series of humming noises produced by waves crashing together and into the ocean floor, off the North-West coast of the US. Potentially, sound from these collisions could travel to many parts of the globe. No mechanism has been suggested to explain how the Hum is heard in the middle of remote land masses, hundreds of miles away from any ocean!

In the case of Kokomo, Indiana, a city with heavy industries, the origin of the hum was thought to have been traced to two sources. The first was a pair of fans in a cooling tower at the local DaimlerChrysler casting plant emitting a 36 Hz tone. The second was an air compressor intake at the Haynes International plant emitting a 10 Hz tone. After those devices were corrected, however, the Hum persisted.