My noise became constant. It would pause a bit right after a shift, but would come back within a few seconds of pedaling. It sounded a lot like those cheap bird calls that you can buy at places like a National Geographic store (
http://audubonbirdcall.com/ ).
I've decided that it's the rear derailleur only after turning my head to the side to try to locate the noise. It seemed like it kept coming from the rear no matter which way I turned my head. It definitely wasn't in the crank, and it would change depending on chain speed, not wheel or cassette speed.
I'm usually pretty good at figuring out which direction a noise is coming from, but this one still took a while.
