I bought a 2010 F250 that had the same problem. The dealership said it was strange and after trying several fixes, finally deferred to Detriot to diagnose. After six trips back to the dealership, they eventually decided it was "normal" even though they originally agreed it was not. When I mentioned invoking the Lemon Law, they bought the truck back and gave me a sweet deal on a 2011 F350 Dually. The new truck has 18,000 miles on it and has not yet developed the "normal" clunk. Unless your dealer has an extraordinary service department, you will be in for continued clunks.