Loud buzzing on ignition.. Ideal Mexico Floorstanding Boiler?

A friends boiler emits a loud buzzing (sounds electrical) when the boiler is trying to light, sometimes for a few seconds, sometimes continuously, though its not every time making fault finding difficult..

I'm guessing its a contactor/relay on the PCB, or the solenoid on the gas valve.. when the buzzing stops it lights)

Anyone like to guess which is the most likely..?