In May I replaced the failing and ancient central heat/air unit in my 1940 era house. The house is well built and the ductwork drew compliments from several of the contractors who bid on the job. I picked a well established contractor who completed a Manual J heat gain/loss calc on the house. Since I plane to live here a long time, I choose a high end heat pump unit (93%) & 15 SEER from RUUD, the UPRL series. The installation went fine and the unit is cooling the house well enough.

But there is this high frequency hissing/static type noise that is still making this unit unsatisfactory. The VS fan is fine on the low setting with no noise, but as the fan comes up to cooling load, heating load or just high fan speed, the hissing that comes from the ducts gets into my ears and wont go away. The ducts closest to the indoor unit are the loudest and the sound fades the longer the ducts are. The contractor guys have been good to change out the fan, the aux. heat exchanger, and the evap coil, but still the same hissing. They have looked over all the ductwork and dont find a source of the hissing. The regional distributor calibrated the temp. and air flow and still the hissing is there.

A factory rep came out and looked things over and asked relevant questions of the tech guys and concluded that the sound is coming from turbulence in the new unit. He tells me that all the makers of the high efficiency units use a similar design and that this noise is part of this type of equipment. His idea was to install some flex duct to the short runs to dampen the sound.

I am wondering whether to start over with new equipment from another maker, (but will I still end up with similar hissing?) or to try some of the flex ducting or what about duct liners? Can liner be installed in the existing ducts or not and is the liner safe?

Any informed opinions on this situation would be greatly appreciated. Thank you.