I would think about the timing chain adjuster in this situation. Alos, the sound is indicitive of poor carb tuning. Start there, if it was an internal engine issue you would hear it at all times, and a bearing would have failed and locked your rear up rather quickly. Best bet is the timing chain adjuster, when that is not working correctly it allows excess slack in the T. Chain which will cause the sounds your describing. You should look into this before riding anymore because this can lead to catastrophic engine damage if your timing goes out at high revs. We are talking bent valves and possible engine lock up. An Ape Manual chain tensioner will solve this issue if the adjuster is shot, but investigate before throwing parts at the problem.

 

For my bike, theclicking noise during accelerationturned out to be the cam chain tensioner. I ended up buying a manual tensioner and installed it myselfin about five minutes. Worked great! On occasion I'll hear the noise again. When I do, I just tighten it a bit and I'm good to go. Hope that helps!