Fortress Hays

This morning when I arrived at work I was greeted by the sounds of a woodpecker busily searching for its breakfast. As luck would have it, I had my voice recorder handy, so I was able to capture about two and a half minutes of the tree’s agony. A truck goes by in the middle of it and an assortment of other birds chime in, too. As I was converting the audio to MP3 format I noticed a curious thing - the decibel levels of the pecking show some distinct groupings. I wonder if an ornithologist could offer any insight?