I have a 2000 YZ426F. had the valves shimmed a couple months ago when I bought it and it has been running great. Still does run great and start easily, but I'm starting to hear motor noise. its a tapping sound that almost sounds like the skid plate is loose but Ive checked everything and nothing is loose. The sound speeds up with rpm's and sounds tobe coming from the middle to top of the motor. I've taken the valve cover off and checked the bolts that hold the cams and theyre snug. the gears and chain looked good as well I couldn't see anything unusual. Any suggestions what to look for? I'm not going to ride it again till this is fixed.