The damper door, about the size of a playing deck card, makes a clicking/vibrating noise for about 5 seconds when that door opens. This door opens to let cold air into the frig from the freezer side. This noise started about 1-2 months ago, and remains consistent, and constant. There is no decrease in the cooling capacity of the freezer or frig. Indeed, everything appears to be operating properly, other than this vibrating noise. The other day, while cleaning the refrigerator, I actually observed the door vibrating after it opened, so I know where it is coming from. There are no obstructions anywhere, and the door itself appears to be solid and hinged properly. Any ideas?