Cool video Mike. So is the sound in the cat, the same sound you were talking about in your original post or something different?
Since you have a welded setup, it'll be tough to take off the cat and shake out whatever is rattling in there, which could just be welding slag or the honeycomb structure itself.
The other noise sounds like heat shield rattling or exhaust pipe somewhere, although the exhaust gas pressure could be causing whatever is in the cat to rattle.
BTW, why aren't you using the y-pipes? Take those blocking plates off, LOL!
2011 Race Red GT vert, 5.0 A6
FRPP ProCal, JLT oil separator, Boss 302 quads w/ Corsa Xtreme axle backs, Koni STR.T (orange), GT500 strut mounts, Steeda Sports, Steeda billet LCAs, Steeda adj UCA, Steeda adj rear swaybar, BMR adj panhard, BMR lca relocation brackets, Boss 302 lower front fascia