A motor's case provides significant magnetic shielding, though induction cannot be ruled out as a source of noise, particularly of there are untwisted wires driving the motor and/or signal conductors near the motor and/or its power wiring. The more likely source of noise is from the motor's brushes (if applicable to your motor). A capacitor connected close to the motor's terminals is often used to suppress brush noise (say .22uF). It is a good idea to twist the two wires together that drive the motor.