I got out my Peterson's and thought from the description that I must
have heard a Chestnut-Sided Warbler.
Chestnut-Sided Warbler = please please pleased to meet cha, the
penultimate note accented, last note dropping. Also a more rambling song.
It's the only warbler description that seems to fit what I
heard: treat treat treeeeeee tree'-et
(the second to last note slightly higher than the rest).
But then I listened to the song online and it didn't sound the same
as what I'd heard, as this was much more slurred than I had heard and
ended a bit differently. But, Dana, your most recent description
fits EXACTLY what I heard, so we really do need to figure this
out! What fun! (Evergreen...wanna come listen?)
P.S.: Here is a website where folks can listen to the song of
various birds to help with identification.