For decades, hundreds of people worldwide have been plagued by an elusive buzzing noise known as "the Hum". Some have blamed gas pipes or power lines, others think their ears are faulty. A few even think sinister forces could be at work.
"It's a kind of torture, sometimes you just want to scream," exclaims retired head teacher Katie Jacques.
Sitting in the living room of her home in the suburbs of Leeds, the 69-year-old grandmother describes the dull drone she says is making her life a misery.
Most visitors hear nothing, but to Katie the noise is painful, vivid and constant.
"It has a rhythm to it - it goes up and down. It sounds almost like a diesel car idling in the distance and you want to go and ask somebody to switch the engine off - and you can't."
Katie says she no longer has any quiet moments and getting a good night's sleep has become impossible.
"It's worst at night. It's hard to get off to sleep because I hear this throbbing sound in the background and you know what it's like when you can't get to sleep and you're tossing and turning and you get more and more agitated about it."