but I'm not Charlie from wmass so I'll give you a decent answer. If you replaced the ignitor, the electrodes should still be adjusted where they were before, but that could cause it. A burner setup with instruments would be in order also since improper fuel pressure and air adjustment can cause your problem also. Efficiency will suffer if the burner is not adjusted properly too.
Rumbling can be the result of insufficient gas supply. Check the firing rate if able to clock meter on natural gas. If over-firing, you may lose ignitors. If you can't clock the meter, Lp C02 values would be about 10%CO2. Natural gas CO2 is 8.5%
Might be a good idea to check the condition of the burner. Is the condensate drain built to manufacturers specs. It should be located on the boiler tee at side of boiler. Coming up outof the trap should be 3.5" rise and trap should contain water.
Have a 2 yr. old Utica UB-90 200 that developed a crack in the igniter. Replaced it and now just as it ignites on start up there's a slight rumble that lasts a very short time but can be felt on the first floor when the boiler is in the basement. I have no idea what may be causing this.