murmuring trees

When I first moved to the mountain and would take my morning walk, I often heard a bird song that had the cadence, "trees, trees, murmuring trees." After some time spent trying to hum it for various people on the phone, I resorted to listening to Birding by Ear in its entirety (luckily I have a long commute). I soon found out that it was a Black-Throated Green Warbler. They nest up here. Of course, I had seen them before during both spring and fall migration, but I had never heard the endless murmuring of the trees.