My '01 s60's brake pedal is behaving strangely. The car is new to me so I'm not sure if this is normal. At first it emitted a high-pitched whistle, an ethereal sound that was hard to trace back to its source. That seems to have stopped, but now it often gives off the hissing sound of air escaping, as if it's depressurizing. Both these sounds happen after my foot is taken off the pedal, and last a good 20 seconds. Mild pressure on the pedal stops the noise. pb