Larry in Calgary has the problem that the wind makes a lot of noise with the damper on his bathroom exhaust fan.
The banging noise of a duct damper can be reduced or eliminated by carefully placing the self adhesive foam insulation strips as soft bumpers against the damper. Make sure that the damper still swings free, but stops on the soft foam when it wants to slam shut.
We also showed a special roof exhaust trap for fan outlets. I always prefer taking exhaust fans out the gable end of a roof, on the vertical wall surface, because it allows for a downward drainage and no chance of a roof leak. But when you must go out through the roof deck, the vent shown in the picture gives about the most wind resistant hood we can put on a roof. It is even designed so that any condensation in the trap will drain out onto the roof rather than back into the attic.
The vent in the picture is made by Ventilation Maximum from Quebec and is called a "Maximum Roof Ventilation Trap - CT-4 or CT-6"