noise in boiler

Chris B._2

I have a Weil-Mclain gold boiler approx. five yrs. old with a Riello burner. After the burner fires and has been burning approx. one to two minutes a low pitch humming noise can be heard in the boiler which gradually increase in intensity and can be heard in the rooms above. When the burner stops the humming noise continues for about ten seconds or so and decreases in intensity and stops. A sizzling noise can also be heard at this time which also stops. Any thoughts on what this may be? The humming noise is there whether the circulators are running or not. I converted the Riello from a two line to one line system, no help. The boiler pressure is fourteen psi and the temperature is 150 to 180. This is a three loop baseboard system with a indirect water heater.

herb_4

sounds like a prob with the ignition when it finally drops out which riello's do the sound subsides. might be in the control.