buzzing boiler?

can anyone help! i have a baxi back boiler with electronic pilot igniter,when it tries to light,instead of lighting it makes a very loud buzzing sound.I have never heard anything like it before or since.If you turn off the petenchiometer,then turn it on again and sometimes it lights,then sometimes it doesn't.I have replaced the petenchiometer and had the boiler serviced but to no avail.i can find nothing wrong with it neither can six other people that have looked at it,but the problem is still there.any help please.