One problem related to brake whining noise has been reported for the 2012 Mazda CX9. The most recently reported issues are listed below.
Brake failure at 60k miles (possibly earlier). I bought the vehicle from a dealer 2 months ago and was told front and rear pads and rotors had already been replaced. They felt inconsistent but I thought they needed to settle in. Then the power boost began to fail and a high pitched whining noise comes from the booster if the pedal is held down. It has lurched forward with my footrr on the pedal while at a full stop. I now put it in neutral when at a stop. Getting it to stop is the problem. Very ,very dangerous.