We are disappointed to learn of your experience with the WVK2A wall venting kit. We can recommend adding a piece of foam or felt at the point where the flapper hits the damper. This can help to soften the noise you are hearing. Alternatively, relocating the cap so it doesn't face prevailing winds can be a secondary option.
The wind is something that just cannot be controlled and on extremely windy or stormy days, you may still experience this situation.
We do suggest that you contact our Technical Support Department should you have further questions or concerns. They can be reached at 1-800-637-1453. Technical is available Monday through Friday, 8:00 am until 5:00 pm CST.