alpha cb24x

[gbarker]

Hi I have an Alpha cb24x boiler that makes a loud humming noise when the heating first starts, this can go on for a couple of minutes then the boiler runs fine and smooth.
All the radiators are hot to touch with no cold spots and the water heats up as normal.
I did have a leak to which I added some sentinel express leak sealer which has fixed it for now, but this noise appeared after I added it, I was told to try some fernox noise reducer which I added a week ago but has made no difference to the noise.
I know its olds and not the best boiler but it hasn't let me down up to now, does anyone have any ideas what this could be before I start spending loads on a boiler when its not worth it.
thanks.

 

[durhamplumber]

loud humming....before the burner lights?... could be fan before it starts spinning or solenoid on gas valve..if continues after burner lit i would guess gas valve

 

[gbarker]

You hear the burner light and then the noise starts only for 2-4 mins and then runs smooth and quiet.
The only way I can describe it is an aircraft powering down.