I have two packaged units for my house. One is right outside the guest bedroom. Guests have let me know that it's very loud. It even bothers me at night. Although the unit itself is loud, the bigger problem seems to be more from the vibration of the sheet metal ductwork that runs between the unit and the house. Is there some kind of rubber coupler that would dampen the vibrations? And if so, approximately how much would it cost to add one after the fact?