What it was
The dealer just called. Evidently, I have a "torn bushing on a sway bar link." Is that weird, or what? I swore to him that I hadn't been autocrossing, and I haven't. He said they did a thorough inspection of the area affected by the bad bushing. The rattling I was hearing is the sound of metal on metal, but he said there's no damage.
They couldn't find another bushing anywhere in Arizona, so they're having one overnighted. Considering it's Friday, it'll be Monday before they can do the repair. I'm free to take my car home for the weekend. Weird.