I have an 08 yz450 and after putting the motor back together it made the same sound. I rechecked my cam timing and I was off by one tooth on each cam causing the valves to touch the piston at tdc. I originally put it together using the wrong mark on the flywheel. Luckly it didnt do any damage and the bike is back to normal now. I've also seen where the timing chains are worn or stretched and can skip teeth on the cams doing the same thing. That would be something easy and worth checking.