My '00 WR rattled too when it was new. The noise went away after about 50 miles. This was immediately after added a 45 PJ and 170 MJ and giving the bike an oil change. After changing the YZ timing, it rattled again for about 2 rides, again until the jetting was further dialed in. If the noise is a high pitched, tin can (chink, chink, chink type sound) type sound, you may be a little lean. If it's more like shaking a paint can sound, I bet there's a loose bolt somewhere or it's just normal. Also, before I rode the bike, I greased the bejesus out of everything in addition to tightening ALL bolts. There where a few engine mount bolts on the loose side as well as the header, tank, linkage, and carb boots. Tighten everything down, I bet that will help. The YZ/WR's do sound different than a typical 4 stroke.