While watching Letterman, I heard and "felt" a low hum. I muted the TV, and could still hear the sound. I opened the sliding glass door, and the sound was definitely coming from outside. It was a clear night, with a sky full of stars. The house is in rural PA, so there was no light pollution to contend with. Standing in the driveway, I scanned the viewable sky and didn't see anything, but could still hear and feel the hum. It sounded like the hum was coming from beyond the tree line to the east. About that time, I noticed the the sky from that direction was becoming "blacked out" (ie the stars were being blocked by an object passing between my line of sight and the sky). The object continued across the sky from east to west, at a slow rate of speed. As it cleared the tree line, I could clearly see it was triangular in shape, and it continued to block out the stars as it moved directly overhead. There was a dim glow at each point of the triangle. I ran inside to get a camera, returned and took several pictures. None of them developed. It disappeared over the treeline to the west, and the hum diminished as the object moved further away. It was a large object, but it was difficult to estimate it's actual size or distance from the ground.