Occasionally the stop pin rattles a tiny bit when you get a tolerance build up. Max hole size combined with min.pin end diameter. This has no adverse effect on the function of the knife. The stop pin is designed to rotate to avoid wear over time.

If it really bothers you, a small drop of blue Loctite will stop your rattling stop pin. Put a drop on a toothpick and then onto the stop pin allowing it to migrate into the hole. It will set up in the hole in the liner and will stop the rattle.