N reg. I think that is the version where the rear bumper comes loose (did on my R reg). You may have noticed how many Mondeo rear bumpers are split/taped up. This is because a little plastic clip on the bumper fits on to a metal bracket (to stop the bumper 'wobbling'), but it only pushes on. The 'wind' can unclip it. If yours has become unclipped, it could be rattling against the back end. Solution is to refit the clip onto the bracket, then drill a hole through both and use a self tapping screw to secure it properly.