I used to have a 180mm shimano XT rear rotor and I just went up to a 203mm shimano XT rear rotor. Brake caliper is Shimano SLX. The 180mm was nice and quiet. The 203mm moans when braking. It sounds like rotors sound when they get wet. The wheel spins freely when not braking, there' no rubbing, the caliper looks aligned, the brake pads are fairly new. Any suggestions?