What is this Sound (Video)

[reconchris]

The video is below. You can really hear the sound during deceleration. It started all of the sudden the other day on my way to work.
https://youtu.be/fR3fj8Bkk8U

So went over all the valves and and checked the chain. Some of the valves were lose but that is because I set them that way 9 months ago. This sound started 2 days ago. I "tightened" some of the valves and made sure the locking nut was very secure on all valves, then rechecked the clearance again. I then did a visual of the chain and found it to be normal. It was too dark to check the timing but I think that will probably be fine too. I don't feel like I fixed anything. I was looking for a smoking gun when I took the cover off but didn't really see much. With the sound, the engine performance and idle all seem normal there is just a very aggressive knock. Will do a small test ride in the morning to see if the sound comes back.