Olympian Washington has a history of odd hums. A recent article in the Olympian of an report of an unidentified hum in Olympia, Washington mentions previous hums as well, with more in the comments:
I remember this sound. I heard it every night off of Oyster Bay Road, easily over 10 miles NW of Olympia. We never figured out where it came from. It is a very rural area with none of the things like street sweeping or pressure washing. I remember we thought maybe it was coming from further north and that the sound was more audible because the water amplified it. I guess it could be similar in pitch and frequency to the sound of the trains traveling on the tracks (which I can hear in Lacey).
For more info on the Hum world wide, check out thehum.info