This was in Haywood county, NC. I lived midway up a mountain (Cold Mountain, same as the book/movie). My backyard was thousands of acres of wilderness, and I'd often spend my days hiking up to the closest ridge and back down. A friend joined me one day, it was mid-Spring. The hike up was uneventful as always, but on the way back, both of us stopped when we heard a very low but LOUD gutteral growl. We both looked around, and asked each other if we'd heard it. It seemed to come from no direction in particular. After a long pause, we continued on.
We made it only 5 feet before again, we were stopped. This time we didn't hear this, we FELT it. This long lasting growl punched right through both of our chests. You've never seen two people make it down a mountain in such record time as we both did. Picture that cheese rolling race in England, but we're flying through the mountain laurel, half on our feet, half on our butts as we made the quarter mile trip to my yard.
There are no animals in those mountains that could do this. The only one that could have a remote chance is a bear, after a talk to the locals (including the ones that hunt bear), that was ruled out.
My friend and I were pretty well shaken up the rest of the day, but it didn't keep me from hiking again, I was just maybe a little more mindful of my surroundings.