Gray ovals moving down a tunnel

Sometimes, when I look at the sky(and sometimes when I'm not), I see gray ovals/circles moving quickly down a tunnel to where I'm looking. They get smaller and smaller as they move down the tunnel and the tunnel seems perfectly circular. The circles seem to make up the tunnel actually, not be inside of the tunnel. There's a small empty space in the middle of the tunnel. It takes up a small part of my vision and its not annoying. Its actually kind of interesting to watch them, but I was just wondering what this was called. I can't seem to find anything about it on the internet. I don't have eye problems, i got my eyesight checked recently and i have 20/20 vision, but I forgot to mention it. I can't remember when it started happening, but its at been at least a few years if not my whole life.