Not sure if anyone was following this but finally got the dealer to ride with me to listen to it.
Turns out it's something rattling in the metal flex part of the stock downpipe. They are ordering a new downpipe for me and installing it under warranty of course.
I asked the guy how much the stock downpipe cost, and he said like $1700. I laughed and told him APR's whole turboback system is like $1300.
Oh well, for once, modding my car with an aftermarket exhaust would have actually fixed something! But alas, the new TBE prob won't be for another 6 months or so.