I don't have the AAM Short Tails, but I believe I have the same Rattle IN the hatch as the OP.
I've torn apart my hatch interior multiple times as well trying to figure out what it is. So far I have narrowed it down to the 3rd brake light. It has 2 posts that stick out to align it when you're putting it into the hatch. These posts go through small alignment holes inside the hatch. If it isn't centered (which they never are) they will vibrate as the exhaust goes through the 2-3k resonance band. Major aggravation ensues.
So far I have installed rubber grommets in the holes to center the posts and prevent them from moving. So far this has prevented the "metallic" sound, but the posts still vibrate. So on to step 2. I have purchased some dense foam that I will try to stick on o the hatch interior trim, and when reinstalling, should brace the posts and prevent their movement, or at least muffle it to when you can't hear them anymore.
I'll be doing this over this weekend so I'll make sure I take video and pictures to help the rest out if it works...
