Every s197 mustang i have ever spent real time in (4 total) have had a rare rattle from behind the glove box at speed.I removed the glove box and traced the rattle (more of a tick/pop) to the lower support bar. I put some velcro pieces between the platic dash part that the glove box attaches to and the metal support bar (also has a ton of electrical grounds attached) and reassembled.
It worked for about 4 miles than it came back, then a bit later it went away again. Really annoying sound.
I am not sure I want the dealer trying to "fix it" as they may scratch the dash or create another rattle.
I am going to buy some foam tape and insulate the hell out of the area which I am sure is just what the dealer would do anyone.
185 miles on the car and it was down to a 1/4 tank left so yes I am enjoying it! Only complaints are the automatic trans (I miss stick) and the glovebox tick/rattle.