Why is 'the hum' such a mystery?

A village in Durham is the latest place to report a strange vibrating noise - known as "the hum". Why is it such a mystery?

This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.

Source: http://www.bbc.co.uk/go/rss/int/news/-/news/magazine-13752688

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