At about 70deg the burner pulses in my combi boiler and this makes a vibrating noise in the fan,any ideas?

im a gas safe engineer and please don't muck around with the boiler get a competent person in . as for the pulsing this isn't normal, but it may pulse when you have the cover off, as it is a room sealed boiler. please get a proper gas engineer in to look at your boiler before you do some damage.