I have been hearing a sound ever since I got this bike and I could never pinpoint what it was. I think I may have it figured out. When I spin my back tire there is the normal clicking sound from the cassette. but if you listen closely there is also a click sound every revolution. You can really hear it as the wheel slows down. When I sight down the drive train while the wheel is spinning, there is a slight wobble in my cassette consistent with this clicking sound. Just my preliminary diagnosis, but does my skewer have a bend in it?
It seems to me that it could be causing the sound/wobble?

I shot a video. You really can't see the wobble very well....maybe a little toward the end if you really focus on it. But you can hear the clicking very well.

Video Here