Risking the Team Sgt's wrath, as Im neither SF nor work for FN, but I work in an armory full of SCAR's.
I would go out on a limb here and say that the odd pinging your hearing is from that craptastick Flash hider/suppressor mount that it seems is simply tighted down with a wrench, without even loctight, let alone rockset which is IMHO the only way to secure something that is holding a supressor on the end of a gun.