what kind of bird is this

I heard the jays yelling first. As they moved away I heard another bird calling. It sounded like a loud whisper, a bit harsh but with two distinct syllables. I walked to the door to look out into the trees. What bird is this? I stood still, and listened. I scanned the leaves and branches, moving in the morning sun and breeze. I relaxed and listened. This bird was saying, "KeepOn, KeepOn, KeepOn"...