Loud buzzing noise when boiler ignites?

I have a Worcester Bosch 24i RSF boiler and it is making a very, very loud buzzing (kind of electrical) noise on ignition. Thought it might have been the electrode as sounds appears to be coming from the combustion chamber but that was changed and still makes the noise, it started out as only for a few seconds on ignition but now it is going on a lot longer. I am still getting hot water and heating but I might add that for the heating to kick in I do need to turn to thermo up very high? and the noise isn't so bad when hot water is turned on either. Can anyone please help ??