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Neighbours of a zoo are being kept awake by a weird rumbling sounds.

But it's not machinery - it could be the noise of giraffes HUMMING.

Residents have complained for over a year of a unusual rumble that was thought be coming from a boiler in the giraffe house at Paignton Zoo, Devon.

A group of 20 even signed a petition urging the council to soundproof it, but the zoo couldn't locate the problem after checking all the equipment.

However, new research suggests the giraffes themselves may be to blame for the hum.

Scientists in America now believe giraffes can communicate in infrasound, at frequencies too low for humans to hear.

They have a larynx, but it is not clear how it is used since the neck is so long.

Zookeepers find it unusual, the research says, that mother giraffes seem to make no sound towards their young so it was believed they could use infrasound frequency instead.

Researchers from the University of Vienna and the Berlin Tierpark zoo collected almost 1,000 hours of audio recordings from zoos in Berlin, Copenhagen and Vienna.

They found the usual snorts and grunts but also found 'harmonic, sustained and frequency-modulated "humming" vocalisations' during night recordings.

(Image: STEVEN BIDDLECOMBE/COMEDY WILDLIFE AWARDS/MERCURY PRESS)

The suggestion is that the humming may be giraffes communicating with one another, and the sounds were only heard at night and in a zoo environment.

Angela Stoger of the University of Vienna, told New Scientist magazine: "I was fascinated, because these signals have a very interesting sound and have a complex acoustic structure."

However, staff at Paignton Zoo raised doubts about whether the new research shed any light on its humming mystery.

A spokesman said: "It's an interesting subject and could shed new light on behaviours and relationships among the species.

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"However, we can see no link between this research and reports of a sound effecting a neighbour last year.

"In over 900 hours of recordings, none of which was made at Paignton Zoo, the scientists found just 65 giraffe hums.

"These were very short calls, lasting a matter of seconds at most, which is not consistent with the sounds reportedly heard by the neighbour."