I had same problem, worse when wind was from certain directions. Warranty dept. quickly told me there was nothing that could be done. The fan does vent into the attic, not through the roof. Inspection revealed that the damper was normally open about 1/2 way and a gust of wind caused it to close with a loud tap. As above, I added a small weight to the damper so as to have it remain in the closed position at rest. For insurance I also placed a small felt pad where the damper hit the housing. All done from in the house. After more than 6 months there has been no tapping at all in either bathroom.
Hope this helps.
Steve
