stick your head under the exhaust and give it shake to see if theres excessive movement, could be loose bracket, careful not to get any muck in your eyes!! make sure your spare tyre is housed correctly and none of the tools are loose. If its more of a deep clunk it could be your rear suspension bushes which could be worn by 60k. also cambelt should be changed at around 60k so get that done, if it goes it can cause massive damage to other components. hope this helps a bit