Did you add any snubbing capacitor across the power terminal of the brush motor to absorb the spark that could occur during the rotation? This not only could resolve the problem but will protect the brush contact in the motor.
If the PLC is not sending out pulses to the stepper controller but yet the stepper motor starts turning when the motor turns, then the noise is actually picked up by the stepper driver and not by the PLC. Most likely the noise is either picked up by the connecting wire between the PLC and the stepper driver or from the power supply. Is the 24V power supplied shared by the PLC, the motor and the stepper driver?
Does your stepper controller have an optocouple to isolate the signal? If the stepper driver uses TTL signal for its control signal then it is more suspectible to noise pick up than optocouple type.