Whooping in Union County, Georgia

I was hiking the Appalachian Trail in July of 2005 and I stopped to spend the night at the Gooch Mountain shelter. There was a hiker there at 16:00 but he left around 18:00 to get in a few more miles. It was a normal night on the AT until around 2:30 in the morning. I was awakened by a very heavy rustling sound. I immediately jumped up in the sleeping bag thinking it was a bear. I was sleeping in the loft upstairs, for just this reason, and I had seen a bear earlier that day. When I shined my headlamp out the front of the shelter there came a tremendous whooping sound. It was a mmoooooopp. It was pitch dark and dead silent except for this tremendous whoop. I was almost painful to the ears it was so loud. I could not see anything. The sound seemed to be about 50 feet away and moved from left to right out of range of my headlamp repeating this loud whoop. There were no other sounds in the woods, nothing, not even the crickets or frogs until 3 or 4 minutes after the sound stopped. I did not sleep the rest of the night. I looked in the morning and found no evidence of a bear, raccoon, or other animal, but the ground was very hard and dry. The sound came from the area where I had my food hanging in a tree away from the shelter. I am a big guy, ex-military and spend a lot of time hiking and surviving in the woods. I have never heard anything close to this sound from any animal I have ever encountered. I do not scare easy but this had me really shaken. I watched a show on The Discovery Channel (I think) a while back and they played a supposed recording of a Bigfoot, then I realized this was exactly what I heard. Until that time I had dismissed the event.