http://faq.ninja250.org/wiki/My_clutch_ ... the_engine.
It is common for the clutch lever to rattle when the engine is running at mid to high RPMs. It doesn't hurt anything, but it can be annoying. You can make shims from a soda bottle or margarine tub lid and slip them in the pivot area like washers to eliminate the noise. Margarine tub lids are softer than soda bottles and may wear out more quickly, so you may wish to use soda bottle material.
Insert the shim(s) over and/or under the lever between it and its holder.
Easy way to make clean shims: cut out a section of soda bottle (2" square or so) and sandwich it between two scraps of wood and clamp. Then use a drill with a 5/16" bit to drill through the first piece of wood and the plastic, into the second piece of wood. Drill a few holes, then remove the plastic and trim around the holes with scissors, making the ring about 1/8" - 3/16" in thickness.
My GPX250 does this too but I didn't isolate it to the clutch lever. However, I noticed when I put my finger on the lever, the rattle stops.