The weird sound, described as a ‘low-frequency noise similar to a police siren’, was first reported in the 1980s in Largs, North Ayrshire.
For decades the sound has baffled residents and investigators have been unable to identify its source.
Various factors have been suggested as the cause, including gas pipes, power lines and low-frequency submarine communications.
And it drove some residents to distraction, with one woman even moving in a caravan to escape the noise.
Some people are able to hear it, while others cannot – and the strange events even inspired a book.
The hum has made a return, 35 years after it was first reported, with one long-term resident of Largs noticing it for the first time.
Andrea Atkinson said: “I first heard it earlier this month and thought it was a car alarm in the distance, but it went on for hours.
“My husband said he couldn’t hear anything, but I couldn’t blot it out.
“I eventually fell asleep and forgot about it until the next night, when it all started up again.
“I only hear it when everything else is switched off.”
She has lived in the town for more than two decades, but was never able to hear it before.
Andrea, who lives in Hill Street, said: “I have lived in Largs for more than 20 years and I remember people talking about it, but I never experienced it until now.”
Back in 1984, resident Peggy Wham first spoke out about the low-pitched droning noise.
And neighbour Georgie Hyslop made headlines after leaving town in a caravan to escape the hum, which she believed was interfering with an electronic implant in her back and made her violently sick.
The phenomenon has been reported in towns and cities across the world from Vancouver in Canada to Auckland in New Zealand.
The mystery even featured in a book called ‘The Hum’ written by American author Ralph Anderson in 2015.
One outlandish theory in the book suggests the hum might be part of a global conspiracy to brainwash and manipulate people.