Got a used Glock 20SF in almost new condition the other day. Loaded up 15 rounds and inserted the mag in the pistol. Nothing in the chamber. There is a rattle when the muzzle is rotated fore / aft (muzzle up, muzzle down) but not side to side. Changed to the other mag and no rattle, so it is mag-related. Any ideas where to start looking when I disassemble the mag?