Since the late 1970s, residents of Bristol have reported a persistent low-frequency hum, strongest at night and often described as resembling a distant diesel engine. Scientific surveys in the 1990s found only about 2% of the population could hear it, but those affected described sleepless nights, headaches, and stress. Despite acoustic studies, no single source has ever been confirmed. The mystery has cemented the Bristol Hum as one of the UK’s most famous unexplained sound phenomena.