Untitled Sighting

I was outside on the back porch at about 8pm. First I heard what sounded like a tree knock in the woods behind the house, to the northeast and perhaps a quarter of a mile away, at most. About 5 minutes later, I heard a sound that started as a low rumble and progressed to what sounded like a deep "whoop". I went inside and turned off the outside lights behind the house. The area is heavily wooded with very low traffic (10 cars on my road would be a heavy traffic day). We are very close to the Lake of the Clouds and several spring heads, with one spring head being located to the east of the house about 1/4 mile away. The wildlife is abundant, with a healthy population of deer, fox, quail and wild turkey. The houses are sparsely placed this high up the mountain, with a lot of wooded space between homes. There are some wildlife trails and four-wheeler trails in the area that the sound came from. The sound was far too deep to be a fox or any other animal that I can think of and did not sound like a deer call.