The “scraping” I heard “in the back of the car” crystallized in my last drive, turning into a very clear rattling noise that I could reproduce at will. I shot this video where the noise is very clearly audible at about 12 seconds in.
With more help from the folks at the VTCI forum, I was able to locate the source of the problem: bad motor mounts.
Yep, both of them.
You can see the cracks in this mount, as well as the displaced bolt–beat out of place by the rocking engine.
New mounts are on the way, along with a few other goodies from the Bird Nest.