There was a thread on this very subject about a year or so ago. After much discussion I think it was agreed that it must be hospital paging, but no one could confirm that's what it was or where it was coming from. It was driving me crazy so I finally locked out the freq. I threatened to sit in some hospital parking lots for awhile and see if I could locate it, but I never did.
I had a fairly strong signal at my house, so I figured it wasn't too far from me, but I soon discovered I had fairly consistent/strong signal over most of the NW Valley, so I don't know.