I am brand new to birding with our 6-year-old son as part of Cub Scouts and I have to start with saying how valuable this site is with information that has easily helped us identify all our backyard feeders so far... thanks! 
Unfortunately none of the suggestions in searching by Washington, unusual, flat, trill etc for the calls are coming up with anything close to the unseen birds we hear for less than a month or so each year that arrived last week. We live in an area of the city with loads of evergreen trees and they're quite good at hiding in there!
They call in a single continuous tone that is sustained for a long time, doesn't really trill or warble, and doesn't vary in pitch except slightly at the end where once they'll drop down a few notes, then with the next call they'll go up a few. It is such a distinctive sound, almost like someone holding down a key on a keyboard set to some strange electronic synthesizer effect.
Does this sound familiar at all to anyone? Every year I've wondered what these are but now I'm on a mission to find the answer!
Thank you,
~Nikki
