I bought a 2005 roadster (33,000 mi) about 2 1/2 weeks ago, and I have a noise similiar to what you describe, only from the right rear. I'm particularly sensitive to noise and vibration, and this is pretty subtle. I thought it might be a brake pad dragging, but had the wheel off yesterday, and the rotor turns easily, and there's no evidence of any scoring of the rotor. It's like a VERY soft grinding noise, and only at low speeds...Is that like yours, Mark?