Since the fridge requires cooling in the back regardless of the power source, propane or 120V, it's probably a 12V fan (or regulated down, even). If it's vibrating the walls, it's either loose, obstructed, or defective (bearings, broken blade). The fridge has a 3 year warranty on it, but I'm not sure if the fan is considered part of that. Have you contacted your dealer for assistance?