First off, how many kilos on the bike? Were any updates ever done such as clutch lifter shaft or cam chain guides? Any other history? Can you hear the noise with the bike standing still? (You initially stated you only hear the noise throttling up from a stop)??? This would eliminate potential brake drag noise/issues. If you only hear the noise with engine running is there any difference with clutch pulled in vs out now? How much freeplay do you have on your clutch cable housing to lever? Should be 2 to 3 mm. Have you noticed any change in this freeplay recently (Indicates a problem)? Is it possible to take a sound clip of the noise?
I'm guessing you should remove the clutch cover and check all the clutch components closely. Lifter piece, lifter arm, basket bearings etc etc. Take a look at the cam chain guides also and measure chain stretch. You might need a shim/spacer in your self adjuster but this would normally give you a paint can type of rattle.