With velocity noise it's the friction of the fluid in the pipe. The higher the velocity the greater the friction. To reduce this you either increase pipe size, or decrease pump speed depending on the parameters you are trying to achieve.
Circulator harmonics is still about friction, and pump / piping solation. Any pump has friction in its moving parts, and the fluid it is moving to some degree. The larger the pump the more harmonics reverberating through the pipes. Isolation of piping from the structure will help.
It should be noted there are different circulator designs. Wet rotor, three piece, ECM .
Most small circus for residential you can hardly tell if they are on.
This post was edited by an admin on February 15, 2013 8:39 AM.