I was walking around Neff's Union Church in Pennsylvania taking pictures of wonderful stones and I heard this knock-knock-knock-knock. Mom, who was in the car at the time, said "Becky! A woodpecker!" I looked up at the tree and there it was. A pretty little woodpecker up in the tree staring at me.
According to AllAboutBirds.org, this is a Downy Woodpecker. You can read all about this type of bird and even listen to how it would sound here.