How old is he?
It doesn't seem urgent if he's playing, eating, and is able to rest comfortably. But, at his next vet visit, I would mention this and possibly get an xray to see what's going on.
Try watching his gait while he's running to see if there looks like an imbalance there. Compare his gait to your other cats' gait(if you have another one). I'm thinking this might be a hip problem.
If the knee flexes normally, I wouldn't worry. Do you think it is a hip issue or a knee issue?