Potterton Netaheat 10 16 - gas valve noise & vibration?

My boiler activates, lights and runs OK, but as soon as the gas lights there is a noisy vibration from behind the control panel and PCB. As best as I can figure it must be coming from the gas valve (assuming the grey square lump that has the gas pipe attached to it is the valve).
The gas flames are a healthy colour and their output is steady, the only problem is the noise and vibration.
This problem is intermittant - as all really annoying problems are!
Assuming the gas valve is basically some form of solenoid I would guess that it is failing to latch and juddering back and fro, but strange that it does not affect the gas flow.
Help please!!!!!!!!!