That metal strip you are talking about is a strip of metal to which the nuts that hold the hitch on are welded to (the nuts are not welded to the inside of the frame rail). It's so you can hold the nuts in place when bolting on the hitch. No other way to reach inside the frame rails to hold them unless you have really long fingers. Both of mine "twang" when I flick them but do not rattle while driving as far as I can tell. You said one was loose. You could be missing one of the bolts in which case the nut inside could be rattling against the frame. Try to wedge something between the strap and the frame to see if it stops the rattle.