May I suggest installing a couple anti-vibration collars. One close to the vent and the other close to the end of the duct. I coated the duct with mastic to keep the air from leaking, and to help insulate the noise. I insulated the duct with rock wool to keep the noise down. I also added a different air leak flap than came with our hood. Our hood is 6 feet wide, and has two electric motors, so they make some noise when I first installed it. After using the items listed above, they are nice and quiet