We were camping in mid Michigan this weekend (Cadillac area) and heard what we think were some strange birdcalls. The campsite was in a dry beech-maple forest with a few small meadows sprinkled in and no creeks or swampy areas nearby. The “call” was a low rumble, which sounded a lot like two tree trunks rubbing together in the wind. We would have let go it at that but we heard the same sound coming from several directions and even when the wind had died down. We’ve camped in this spot for a decade now and have never heard anything like it. We’re guessing it’s birds or toads but frankly haven’t a clue. Can any of you shed some light?