I wanted to clarify something for the guy who called about a quacking sound in his wheel at low speeds whom you advised to get his lug nuts and brake rotors checked. I have heard this sound before and I am willing to bet he has plastic hub caps on his vehicle. If this is the case, I would recommend he remove the hub caps and see if the noise abates (before paying his mechanic to inspect the brake system.)
I want to add that some wheels are made of soft alloy and the carbon black used in tires can abraid the metal where the tire and wheel meet. This sometimes produces a noise due to the loose particles of metal rubbing against the metal.