I borrowed a 1972 FSM and it says the noise could be caused by a defective circuit breaker at the relay or by motor limit switch malfunction.It also says if the motor is over rotated the limit switch will be damaged.The switch plate is replaceable but I doubt it's available anymore,you will probably need a good used motor.I can make copies of the service manual pages and fax them to you if you like.The third wire is a ground conection.