The aircraft was approaching 500 feet above the ground in a climb when the pilot heard a sudden increase in engine noise and dark coloured dust spattered the windshield. He thought that the propeller must have failed and that the engine was overspeeding. He closed the throttle, turned off the ignition and entered a glide. The aircraft was landed straight ahead, into wind but onto downward sloping terrain. The slope was steeper than the pilot had anticipated, and he was forced to induce a ground loop in order to avoid a fence. During the ground loop the right maingear leg and the tail wheel assembly sustained damage. When the pilot subsequently inspected the aircraft he discovered that the muffler had partially detached from the exhaust pipe because a retaining spring had broken. The loud noise had been unmuffled engine noise and the dust on the windshield had emanated from the exhaust pipe.