My '05 WR450 developed a rattle that at first would only rear it's ugly head while riding at higher rpms in 2nd gear or higher. It is difficult to localize the sound when on the bike at those speeds. Fortunately, the rattle eventually revealed itself while idling in neutral while choked. The clutch lever! Just thought I would share that, in case anyone else is trying to do yoga moves on the bike to listen for the rattle.