Folks, I think I may have made some progress on this issue. This evening, I took a drive with some friends and asked them to help me pinpoint the noise. They agreed that it was coming from the front of the car on the driver's side. So I took a look and saw this:
It's a noticeable dent (about half an inch deep) on one of the heat shields. I'm fairly certain that it's not there by design and I have no idea how it got there. But I figure that a half-inch indentation might just be deep enough to make it contact the headers and vibrate during deceleration.
By the way, this is part 22:
I'm not going to muck with this any further, as it's all under warranty, but this is giving me hope that it's just a simple heat shield issue. Anyway, I just wanted to document my findings here with the hope that it might help others with similar problems. More info to come when I visit my dealer on Monday...