91 ford probe windshield wiper problem?

On your car the wipers will retract under the hood when you turn them off (down farther than they will wipe when they are on) so thats why it would make sound even when it is "off". it sounds like a problem with the sensor that detects the wiper position, If you can return the part and get another one do that, if not you will have to fix the position sensor on the motor.