it’s a generic name used in cities all over the world that experience the same strange phenomenon, but despite its ubiquity, that name is still a bit spooky: The Hum. A persistent and invasive low-frequency humming noise that is only audible to some people in the area, the cause of most of the world’s hums is still unknown. And for many, even though a source has been identified, it’s unclear why a humming noise is being produced.

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In Kokomo, Indiana, a small, sleepy industrial town in the middle of America, the hum is thought to come from some of the fans in massive cooling towers at the local DaimlerChrysler casting plant. It’s also been suggested that the hum in Kokomo could be originating from another large plant’s air compressor intake.

Wherever it’s coming from, the hum is still described as a frightening mystery by the town’s residents, who have compared the noise to that of the “deep growling of a train idling.” Many have blamed the hum for serious health problems, including joint pain, nausea, diarrhea, fatigue, and headaches. Others say that those people are crazy, they don’t hear anything, and they don’t believe there is such a thing as the hum. People have been complaining about a strange sound, though, ever since 1999.