Hi FitzyKev - sorry only 5 years after your post. I am getting the same issue with my glowom combi. Electrical humming noise when it kicks in from time to time. Stops if I switch it off at the circuit board. Did you get to the bottom of the problem?
Seeking some expert advice.
We have a glow worm boiler - fairly new (3 years old). Last couple of months we notice a electrical humming coming from the main switch board - I think its when the the heating comes on.
If you switch the boiler off from the switchboard the noise stops.
Ive...