i made a custom end cap for my 250ex and it seems 2 be working
the sounds as if a aftermarket pipe is on ur bike
wat u need is to go a local hardware store end by a fine mesh lyk material and 22 gauge steel and something to cut the steel and fine mesh.
slowly cut the back of the exhaust end cap out of the 22 gauge steel
and then cut a hole in the middle with a 2 inch hole cutter tht attachs to a drill and then cut the mesh put it on b4 the end cap
look nice if you do it right