Mine did this until it warmed up for awhile, after I synced the carbs it went away. (I just put 2+2 together, didn't know why the noise went away until now).

 

A thorough tune up including carb synchis the logicalfirst step. The reasoning behind that is two-fold.First , a tune up and synch is easier and lots cheaper than randomly replacing parts. Second, it's amazing what strange noises an out-of-tune engine or out of synch carbs can create. If the noise persists we'dguess either the cam drive chain or alternator/starter drive chain as the next most likely cause. Take either a large screwdriver or a stethescope and try to isolate the noise to determine which is your problem.