The sh*tty rubber diff bush. It cracks and loosens up and creates a slack which is the clunk/shunt you can feel... It can be replaced under warranty if it's covered, which it should be, being only 6000ks old. Just go to the dealership and say the diff bush is rooted and you want a new one put in. If they road test they'll see the problem. They've replaced 1000s of them over the years, but no one can keep up with the changes in diffs that Ford have made with these cars, plus the BF MKII aint all that old, hence not many aftermarket or stronger options. They've changed the diff about 7 times in the past 3 models :o