clicking noise

I have a 2004 kdx220 with only 150 miles on it and yesterday while riding I started hearing a clicking noise. after investigating i located the area of the noise, the clutch and or gearbox, what it was doing was in first gear it was a faster clicking and as i upshifted it got slower and less noise. and also i nuetral you could hear it with or without the clutch in. the bike ran fine and changed gears fine and after about 15 miles it went away? whats up? also you could see the drive sprocket while in nuetral turn back and forth or vibrate back and forth with this noise, now it does not do it since the noise is gone.