For decades, some people around the world have reported hearing a strange, sonorous background tone that others could not.Named 'The Hum', the phenomenon regularly appears on lists of the most perplexing unsolved mysteries, and has even been the topic of documentary investigations.Proposed explanations have ranged from industrial noise to extra-sensitive hearing – but now, a new scientific exploration of several leading theories suggests that the source of The Hum may be a lot closer to home, at least for some people.According to a team led by auditory scientist Bonifaz Baumann of the German Center for Vertigo and Balance Disorders (DSGZ), some reports of The Hum may be a form of underrecognized low-frequency tinnitus.That doesn't mean every report of The Hum has the same cause, but the findings suggest the source may often lie inside the auditory system, rather than in the outside world."Based on our results, although we haven't ruled out cases of physical external sound sources, we suggest that subjective tinnitus in the low-frequency range is often the cause of hearing pulsations of low-frequency sound perceptions," says neuroscientist Markus Drexl of DSGZ and the Norwegian University of Science and Technology.