A little info: I have a High-ranch style of home, 3 heating zones, with fin and tube baseboard convectors. The boiling is a early nineties gas files 2 stage boiler (Raypak H3-0125B, see photos). When I turn the thermostats up and the heat is just kicking on there is a some humming/groaning, I am not sure if its from the circulator coming on and the water getting up to speed through the system or because it's being heated or a combination of the two. THe noise also happened more often when just one zone (living area, not bedrooms or lower level) cycles on. It seems to make noise when the thermostats call for heat and it goes away in a minute or two. I found the manual for the boiler and there is a valve between the inlet and outlet of the boiler and it states: The boiler is supplied as standard with a circulator and a built -in by-pass to insure the required minimum water flow in the boiler. The by-pass on models H-0135 and H-0180 is provided with an adjustable valve that is factory set in the full open position. The handle is shipped loose. The full open position is appropriate for most systems, and insures adequate flow through the boiler. If system flow is inadequate, (indicated by excessive temperature drop through the system) the bypass valve can be throttled slightly. Care must be taken against over throttling which may lead to inadequate flow through the boiler and boiler harmonics (a humming sound from the heat exchanger). If adequate system flow cannot be obtained without causing harmonics, an additional pump is required. The factory mounted circulator will provide adequate water flow for systems designed at a 20Â°F temperature drop, and system pressure drop or head not exceeding that is shown below. On my system the circulator that supposedly came with it is no longer there and there are Taco 007 circulators for each zone. I have tried adjusting the valve between the boiler inlet and outlet and the system still makes noise. It seems like the noise is more prevalent when just the living area zone is on and the other two zone are cold. I have recently replaced the relief valve, feed water regulator, and air eliminator (neither was working properly) and bleed the system and got most if not all the air out. Any ideas as to figuring out the humming? Also I believe this is a two stage boiling with two firing rates (yeah I know both are probably a lot higher then what is needed), but how does it determine with rate to fire at?