the boiler is making a loud buzzing noise when on standby?

the boiler is making a loud buzzing noise when in standby (not used) mode. The noise is coming from the top left side of Britonny Flexiflue - where the fan and printed circuit board are.I suspected the fan so I stopped the fan manually by holding it but the buzzing noise did not stop. I also disconnected the power and moved the fan manually - no noise

What is causing the noise? The PCB? It seems to be too loud to be coming from the PCB

Please help-