Dr Ben Burville spent 17 years trying to become the first person to record them making the noise.
Seals
Matt Lloyd
BBC News
It is one of Europe's "best wildlife secrets" and home to unique and unusual species.
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Steven McKenzie
BBC Scotland Highlands and Islands reporter
Tonnes of fishing equipment are lost or abandoned every year, posing a risk to wildlife at sea and on shore.
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The pup - called Zodiac - is thought to have swum upstream from its North Sea home in search of food.
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More than 7,000 animals in need of help were reported to the RSPCA over Christmas.
Read moreMore than 800 sightings of whales, dolphins and seals are recorded in UK waters this year.
A charity says the deaths are "not acceptable" and urges visitors to keep their distance.
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The total of 3,068 pups have so far been recorded at Blakeney Point on the north Norfolk coast.
Read moreScylla spent two months in a rescue centre after he was found with neck wounds caused by netting.
Volunteers are being asked to "keep a watchful eye" on seal pups on the Isle of Man.
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