Normal
I am almost positive this has nothing to do with the rattle people are hearing but you never know. I noticed a metallic rattle when I'd hit rumble strips. I got under the truck and had two very loose bolts on the furthest back skid plate. It's a small skid plate beyond the larger silver skid plate. I tightened those and the rattle is gone. Like I said I doubt this is your cause but may be worth checking. The looser those bolts get the louder the rattle will be.