Sorry if I'm late with this suggestion. I take it that the leterbox is flapping? I had the same problem. uPVC door. I found that the inside flap is the same as the outside flap. I just swopped them. (You'll have to take off the soft 'insulation' part from the outside one and glue it to the inside one before you switch them). Problem solved, no more noise from flapping.
Pat, May 2012