My Gloworm Ultimate boiler makes a loud buzzing noise when it is off. As soon as it fires up the buzzing noise stops and the boiler works perfectly OK, on heating and hot water (not a combi boiler). It used to just make the buzzing noise for a few seconds when it switched off, but we had a short power cut today and when the power came back on, the boiler started to make the buzzing noise. I have had to switch off the main power supply switch to stop the noise. As soon as I switch the power back on, the buzzing starts again.