Wood knocking and tree shaking noticed while deer hunting witnessed in Oscoda County, Michigan

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Since the 1930's my Granddad and father have hunted the area of Michigan around Mio, from mid state to the Mackinaw Bridge and over to Lake Huron. As I was grew older starting in the 60's, our family often enjoyed camping summers and fall, and archery hunting every fall in the area east of Mio and southeast of McKinley along the Ausable River, and camped near O'Brien Lake. I didn't think about MI until I started researching BF behavior, and remembered the things I heard while hunting. The two times I remember hearing pounding was in the late 80s. In 1986, the family was camped there and archery hunting in October once again. My brother, two brother-in-laws, Dad, and I were once again together archery hunting. We usually spread out around the area pretty well, and were always separated while hunting. This afternoon, while I was just sitting enjoying the beautiful fall day, I heard what I can only describe as the sound of pounding of wood on a tree a ways off into the deeper woods....