Noticed this earlier in the week. When I brake, it makes the same sound as when it has been raining and the brakes have rust on them. This happens only at particular speeds (braking from 20-35mph). At higher speeds, I guess the combination of road and engine noises masks it. At lower speeds, I reckon it's because I don't need to apply the brakes as much.
There appears to be sufficient pad material left. No idea how to check about disc thickness or whether there's anything else causing it.
Suggestions?