What do Taos, N.M.; Vancouver, Canada; Auckland, New Zealand; Bristol, England; and Largs, Scotland all have in common? A select few residents of these diverse cities are victims of the Hum, a constant, low-frequency noise that has affected seemingly random cities around the world over the past several decades.

Some people who can perceive the noise, called “hearers” or “hummers,” say it sounds like a diesel engine idling, according to a Live Science report.

Several scientific investigations into the obnoxious noise have been conducted to no avail. The cause of the Hum and why it only affects certain people in certain areas is still unknown.

Researchers have investigated everything from factories to traffic to determine the cause. Most agree that the phenomenon is real and not caused by mass hysteria. Many people who hear the Hum have had their hearing tested with normal results.

Most Hum victims hear the noise indoors at night. Each “hearer” usually only experiences the Hum in one location, such as his or her home or workplace.

The Hum is reported most often in rural and suburban areas. It generally affects people between 55 and 70 years old and only about 2 percent of the population in a Hum-zone can hear the noise, according to a study by acoustical consultant Geoff Leventhall.

The unfortunate “hearers” are often driven crazy by the constant drone.

“It’s a kind of torture; sometimes, you just want to scream,” Katie Jacques of Leeds, England, told the BBC. Jacques first started hearing the Hum in 2007.

“It’s worst at night,” she added. “It’s hard to get off to sleep because I hear this throbbing sound in the background … You’re tossing and turning, and you get more and more agitated about it.”

Some victims report feeling sick and getting nosebleeds as a result of the noise. One UK victim’s suicide was linked to the Hum, reported the BBC.

This is not the first time the Hum has kept people up at night. The phenomenon was first reported in the 1950s.