I have installed ultra-quiet bathroom exhaust fans. The fans are remarkably quiet but noise from air turbulence in the duct work negates the fan's low-noise advantage. I'm using 6" insulated flex duct with 4' of straight run before the first bend. The total run is about 12' to the roof. No sharp bends in the duct. I've lined the 4' straight section with 6" galvanized duct to provide a smooth interior to that section of duct before the air hits the accordion like interior in the rest of the duct. This didn't provide much improvement. Does anyone make quiet, insulated ducting?