Glow Worn Micron 50FF?

The next thing to change is your boiler engineer.

He has made a diagnosis of noisy fan and has changed it - you pay him for his expertise - not to guess.

The gas valve is a solenoid operated on off valve - hardly likely to make a noise - when he says he thinks it to be the gas valve then that's another guess.

Get yourself a wooden spoon and use it like a stethoscope to track down the source of the noise - your description of vibrating resonance suggests just that - a piece of the casing or exhaust ducting no longer secure.

Did he keep the old fan?

Good luck...
July 2015