such a nice area. in fact i was living in Eugene (sorta close, within driving distance) when my tinnitus started. but any persistent noise will drive ya nuts. you might want to start researching things going on in the area or your region. could be a man made source from some govt or other company project. you might get enough clues to figure it out and then at least you'd know.

as an aside, mine is a higher pitched electronic hissing sound. sounds like it comes from the middle of my head. had a head injury i think's the cause. i get very stressed.

good luck.