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?)

Thanks!  June

P.S.:  Here is a website where folks can listen to the song of 
various birds to help with identification.