I have an issue with my boiler when it is firing were it makes a very loud buzzing/sparking noise. We have had it on an off before and when the boiler was last inspected he said it was due to air so to bleed the valve in the boiler which i have tried but it doesn't seem to have worked. Here is a link to hear what it is like.
I'd guess at the solenoid in the gas valve, which could mean the valve itself or the pcb's not giving the right message to it, or maybe just a bad connection, only a leaned man and his equipment will be able to pin point it,