My 464 seems to be fine, except that when I tilt it forward and back, something slides inside the wooden stock and makes a clunk noise. Mossberg suggested I take off the recoil pad and check the tightness of the bolt inside. Has anyone had this problem? It is in the butt end, and definitely not something in the tubular magazine. I have not taken it apart yet. The gun has never been dropped or banged around.