Growling in Hillsdale County, Michigan

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. Also, when I was a kid, a group of us came around a corner near my house when we saw what we thought was a bear..we only got a small glimpse of the "animal", and I was probably 10 or 11 at the time...so I don't have too vivid a memory about the encounter, but there were 4 or 5 of us in the car, and at least 2 of us were adults and we all agreed that we had seen something. I have ridden the trail several times since the day of the strange sounds, and I always get the feeling of being watched, if I am by myself, I either won't ride that section of trail, or I won't stop at all for the entire 5+ mile section...and I am always looking over my shoulder and all about...the whole area just gives me a weird feeling, and it is so thick in spots that there are several places that something could hide out in...we had even talked about how easy it would be to go in and camp and never see anybody. I wouldn't even think about staying overnight there now. The area is located in Pitssford Michigan, a little town between Hudson and Hillsdale, the county is either Lenawee or Hillsdale...the area in question is about 5 miles from M34, I'm not sure of the name of the side road it's located on.