To anyone interested, a weird thing happened yesterday morning. As I was taking my car in to get my two rear tires changed, I did the head liner check on the way in. In my case, the headliner wasn't the culprit.
The weird part is that after I replaced my rear tires the buzz was nearly gone!
The tires that were on there previously were stock RE050As, but whoever had the car before me sprung for the run-flats. My right rear tire was WAY low, like, 1/32 low. Since the buzz seemed to come from the rear right, I wonder if the harmonic resonance of the tire caused it at certain tire rpms, especially since the run-flats have such stiff sidewalls. My new tires are not run-flats, so softer sidewalls (and holy Moses, what a difference in ride comfort).
My drive home was in a bit of traffic, so I couldn't really give it a full test. I'll keep researching over the next couple days.
Any thoughts? Any similar experiences?