RaineMan: Yeah, this is in fact my first Subaru! In fact, it's also my first new car (all my previous vehicles were purchased used) so I'm extra paranoid. But I figure that's what the internet is for
I have an appointment with my dealer next week, but I suspect they won't give me a completely unbiased opinion. That's why I'm here!
RIPEVO9: It goes away if I push the clutch completely in, but pressing it lightly doesn't seem to have any effect. From what I can tell so far, the intensity of the sound is related to the amount of engine braking and how warm the car is. My suspicion is that it's a resonating heat shield, but who knows?
Anyway, I'll be sure to post whatever I learn from my dealer. Many of the related posts I've read have been regarding older cars with aftermarket parts, new transmissions, etc. Since this is happening on a stock 2011 2.5i with 840 miles, my hope is that a lot of those variables can be eliminated.