I'm always skeptical of advice to put a capacitor in parallel with a motor that is being controlled using PWM. The capacitor causes high peak currents to flow through the switching transistor, mosfet or h-bridge, which may damage it. It may reduce noise coming from the motor, but it will certainly increase switching noise. IMO a capacitor across the motor terminals should only be used in conjunction with an inductor between the motor and whatever is driving it.