We have a ducted and a none ducted AC unit. When the air is all forced into the ducts it's a little quieter but it seems to reduce the total air flow having to pushing all that air through the ducts. In a "smaller" unit with an open floor plan ducting is not required. The longer the trailer gets and the more rooms it's divided into, ducting becomes a requirement to try and get the AC to both ends. Noise seems to be more an issue of the particular brand/model of AC unit. The faster the air flows, the louder they seem to get.