The Airbox bracket(s) can break. These brackets hold the airbox with the carburettor in between to the engine and are subject to engine vibration. The Airbox may hit against the frame of the bike causing undue stress on the bracket, but the bracket may break on its own. This will cause scratching/vibrating noises at different levels of engine speed. To see if one of the brackets have broken gain access to the airbox and move it around checking if the backets holding it are broken.
