Knock, knock, knock…is someone at the door or is that my Solenoid?

I have a LOUD knock when the Solenoid valve closes sometimes. It is happening more often now. I have had Arrow Heating who installed the unit out but they were unable to stop the noise. They turned down the water flow but we still have the problem. The valve closes OK sometimes but other times it makes a loud knock when it closes which can be heard in the whole house. I understand there will be a small knock when it closes. Is there something I can do?

A humidifier may cause a "click" or "bang" sound for one of two reasons.
1) Water, as it moves through the solenoid of the humidifier, can build up pressure and create noise. Usually water hammer is a repeated bang. To see if water pressure is the issue, turn off the water to the solenoid. If the noise stops install a "pipe silencer" or extra coiled water supply line to the humidifier to reduce the noise generated by the pressure.
2) The solenoid itself will create an audible "click" as the humidifier turns on and off. This "click" may be magnified to sound like a "bang", by the ductwork or the proximity of the humidifier to the living space. Usually a solenoid will create only one "bang" when the solenoid opens and one "bang" when the solenoid closes. Isolating the solenoid from the ductwork with foam insulation or remote mounting the solenoid will be necessary. Please note; If the solenoid does produce a repeated "bang" when it opens or when it closes, the wiring should also be looked at to eliminate the repeated opening and closing of the solenoid.

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