Baxi Solo 3 pfl30/80 noisy PCB?

Hi, my Baxi 3solo pfl 30/80 started making a constant humming/buzzing sound when idle (like a noisy old fridge) from the PCD area. If I flicked the switch for heat or water the boiler fired and the noise stopped until put back to idle and it would start again. It is definitely in the PCD area as I removed the PCD housing unit and you can feel the sound resonating through my hand. My question is do I just need to change the whole PCD or is it a component on the PCD. Or does anyone know better.

Kristian
July 2014