I heard a very low but strong rumbling noise and went to the back porch to see what was making it. When I opened the Storm door I looked up and saw a large disc shaped object with a dome on top and a triangular bottom, about fifty feet above the tree tops, which are about fifty feet tall. It was moving very slowly in a steady South South West direction. There were no lights on but I could see it against the skyline. What stood out to me was the sound!! It was very deep steady, powerful propulsion system sound. The feeling the sound left with me was that it was very, very powerful and under no stress. I've been around powerful diesel propulsion systems and none of them were that powerful.