Tapping noise when unit is not running

The likely culprit is from back drafts coming from outside and into the duct work could cause the damper to flutter. Cross-winds can cause this to be more pronounced than typical, but it is limited as to what can be done to address this. The damper by design is free moving and it is tapping up against the collar. If you have a sticky foam tape (weather striping tape-NOT double sided tape) you can place it over the tabs on the collars as this will silence the sound. We would also suggest checking the duct location as it might be in a prone position to receive backdrafts and needs to be adjusted.