You are correct that the reverse unit solenoid can run quieter on DC. That is because the current is not alternating. When operating on AC, the current is alternating, therefor imparting an alternating magnetic field on the solenoid causing it to audibly vibrate the plunger and therefore buzz. It has been suggested that the alternating magnetic field (as opposed to the static one on DC operation) will also prevent the solenoid plunger from getting "permanently" magnetized and therefore becoming sticky. Also, DC operation of the solenoid will run much hotter and could contribute to premature failure. Recently on the forum here, there was a member who operated an uncoupling solenoid designed for AC on DC and it melted the housing!