Re: potterton profile 80e noisy ignition?

In the care home where i work as electrician we have a boiler as above and for a long time it gives a loud report on ignition[woomf sort of sound] casing shakes.
serviced every year,new p c b last year. was told it could be gasket on front casing but present one looks fine. could it be that main gas burner is on at same time as spark? should pilot be on seconds before main burner?