This little item in today's Eugene-Springfield area newspaper, Register Guard:
Mystery humming sound heard at night(source: Register-Guard, City Region section, pB3, April 20, 2014)
OLYMPIA, Wash. -- Some Olympia residents say they have been awakened in the overnight hours by a mysterious humming sound. The Olympian reports not everyone can hear it and the sound is intermittent. Attempts to track down the um have been unsuccessful, although idling trains, pressure washers and heat pumps have been suspected. -- News service reports
Of course these stories of hums heard world wide, including the in the U.S., have been around for some time in the Fortean arena. I always find it interesting when anomalous events like this are casually inserted as fillers in main stream media.