Brakes whistling then grinding?

Hi... My 2000 A4's brakes (right rear, I think) developed the whistling noise that a lot of Audis apparently develop. According to folks at audiworld.com, it's a pretty common thing as the brake rotors become glazed with crud from the brake pads. Under light brake pressure, you could hear a high-pitched whistling sound. Not every time, but fairly often. With more firm brake pressure, the sound went away. The car's braking performance was never affected, but it was annoying. Now, at very low speeds, after the whistle, there's a really low, grinding noise. At higher speeds, if I really mash on the brakes, it doesn't do it. It's only at lower (around town) speeds, and not every time. It's definitely coming from the right rear wheel though. I took the car to the shop right after it started the whistling noise, and they checked the brakes and said I still had 60% left. Anyway, what is the grinding noise now, and is it time to be concerned? Thanks CW