It creeps in slowly in the dark of night, and once inside, it almost never goes away. It's known as the Hum, a steady, droning sound that's heard in places as disparate as Taos, N.M.; Bristol, England; and Largs, Scotland. But what causes the Hum, and why it only affects a small percentage of the population in certain areas, remain a mystery, despite a number of scientific investigations. Read More at NBC News.com.
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