Well thats a good question as the proper function of the valve is to "buzz". You usually don't hear it but the valve is controlled by what is called pulse-width modulation. The means that the valve is openned and closed at an exact frequency. It is openned and closed by changing the duty cycle. The duty cycle is the relative time the valve is closed versus open. I have always presumed that although the valve receives the open closed siganl completely at a large frequency that the valve actually kind of hovered at a constant spot; sort of the average of the resulting strength of the pulsed signal. The valve may be dampened internally to keep it from actually moving as fast as the signal. If the dampening is missing the valve might actually move as fast as it is signalled. This would in effect do the same thing.
I think that I would have to hear it to do better than that.
Bosch Master, ASE Master, L1
33 years MB technician