Well, this was a very quick fix/solution. I got the car up on my lift and noticed that the right-angle-shaped right rear plastic brake cooling duct had partially detached from the lower control arm thingy (not the actual part names I'm sure). It's held in place by a plastic bracket that goes over the top of the control arm and is secured to the duct by three plastic tabs in the front, and two Torx screws in the back. All three tabs had come undone, and the bracket was just barely holding on to the control arm. Because it was out of place, the wheel-side of the duct was rubbing against the inside of the rim and hitting against the tire balance weights with each revolution. I unscrewed the Torx screws, put the tabs back in place, and refitted the duct. Voila! Quick test drive and noise is gone. I'm guessing that our German friends designed the tabs so that they would break free in the event you hit road debris/speed bump or something. The duct seems like its almost meant to be a disposable part as it's the lowest point of the rear suspension assembly, hanging in the airflow under the car. Anyway, if anyone else hears a weird noise coming from a rear wheel, check to see that these ducts are in place and tight.
