My brakes have started making a bad noise. Under light pedal pressure, there is no noise, and they stop straight. When you use them a little bit harder, there is a grinding noise, much like gears grinding on a non synchronized transmission (how old do you have to be to remember that sound). They always stop straight, the noise is there if you use both brakes or just one. Is there a wear indicator on these brakes, or am I grinding metal on metal?