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.

Thanks! I was about to do the "wedge" treatment... I don't think it's "loose" with a bolt or nut missing (though I will check this); I think it's just bent such that it's slightly elevated... I think I may drop the hitch to get a better picture of this...


However, based on other posters, I think the major noise is coming from my... um, what did he call it? Something to do with a brake proportioning valve... I'm still trying to figure out how that (proportioning valve) would work...
Lateral movement (i.e. track bar moving relative to the frame) causes the unit to change the brake "signal" or pressure from one side to another?

It could be the hitch/tow hook bolt bracket but it could also be the e-brake cable linkage. They sometimes rattle when the rear axle goes over bumps.