We have added onto our house a master bedroom/bathroom of about 500 sq ft. The floors are hardwood and the ceiling is vaulted. (So there is a bit of an echo in the room.) We added a 2nd air conditioner just for this room. The contractor put the air handler unit in the attic and the air return is toward the top of the ceiling (no other place to put it now that construction is done.) The unit works great with one exception - startup and stop. A characteristic hissing through the ?"needle"? valve is very loud for about 20 seconds on startup and about 20 seconds after the unit shuts down. I was told this is normal, but the echo is really bad due to the acustics of the room. The air return is a box made of wood, and I was wondering if I could line it with anything to absorb the sound, but not restrict air flow (and not fall apart to clog the system either). Any suggestions? It is so loud we really hate to even use it right now
That hiss is the pressures equalizing. It is a normal sound, but it is not normal for you to notice it. You just got "lucky" and the duct design along with your room's acoustics didn't compliment each other. You could try adding duct liner to the first sections of supply and return and see if that helps. There's no simple solution to this one. It might take some trial and error on your part.