I would like to call attention to the central unit itself, the working part of the system. This is often located on the same floor level, and even worse if situated near the listening room.
Since this houses the electronics, blower motor and fan assembly, it is a major source of mechanical noise and vibration. Installing sound absorbing fiberglass within the walls of this utility closet will go a long way toward killing these noises.
I went one step farther. Had a custom (extra thick) solid core exterior door and frame with threshold built for my utility closet. It was then weather-stripped with the rubber gasket system for severe weather. Made an unbelievable difference by closing up the last cracks and vents that leaked noise into the space.
The previous suggestions coupled with mine can reduce the various sources of noise to almost inaudible levels.
Since this houses the electronics, blower motor and fan assembly, it is a major source of mechanical noise and vibration. Installing sound absorbing fiberglass within the walls of this utility closet will go a long way toward killing these noises.
I went one step farther. Had a custom (extra thick) solid core exterior door and frame with threshold built for my utility closet. It was then weather-stripped with the rubber gasket system for severe weather. Made an unbelievable difference by closing up the last cracks and vents that leaked noise into the space.
The previous suggestions coupled with mine can reduce the various sources of noise to almost inaudible levels.