LAARS MFTCW, MFTHW, FTCF , FTHF-80,000, 100,000, 140,000. 199,000 BTU's.
A "HARMONIC", "HUMMING" noise may be noticed when the internal primary/secondary pump is running. This noise can radiate/disperse thru out the home and can be very annoying, even though primarily harmless.
This is due to the speed selected on the internal primary/secondary pump incorrectly. The pump is a three speed pump and when THIRD speed is selected in smaller foot print piping (the manifold, header) being very near to the boiler and OR undersized, this "hum"," harmonic" noise/sound may be detected.
Homeowners and contractors have sometimes described/explained the sound to be a vibration or a hum.
In most installations and applications THIRD speed is USUALLY NOT NORMALLY required.
Pump speed selection of FIRST speed or even SECOND speed has shown proper/adequate for MOST all applications.