Two bikers hear strange sound near "Lost Nation" witnessed in Hillsdale County, Michigan

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We were mountain biking through "Lost Nation", state land that is open to most outdoor activity. The land is hilly and fairly heavily wooded, has a river run through it, and covers a large expanse of land, so there are many remote areas inaccessible to most casual adventurers. We were riding up the hill leading into the deeper part of the trail, when we stopped to rest, just as we stopped we heard a deep growling kind of sound that was so loud, you could kind of feel it even though the sound came from a good distance away. My friend and I were at a loss to be able to explain the sound...it was as if something was letting us know it was there...but we never saw anything. After a couple of minutes, the loud "growl" was heard again...just about as close as the first time. My buddy and I just joked it off as being "bigfoot", but I have lived near that area almost all my life, and I heard stories of people seeing something strange out there since I was a small kid...I am now 38 years old. ...