Seattle's mystery hum

Residents of West Seattle are reporting on the return of a mysterious hum that was first heard several years ago. Apparently, the hum shifts in frequency, goes away and then returns. The West Seattle Blog, that first mentioned the intermittent hum back in 2009, has posted a recording of it. The site is investigating whether it's industrial noise from a nearby plant. So far though, the source is unknown. Perhaps the West Seattle hum is participating in a hum symphony with the Taos Hum, New Zealand Hum, and the Bristol Hum. "Is ‘the hum’ industrial noise? Many reports, few complaints – so far"