@Das Boat: I know exactly what you mean (at least i think).
My bike also has it sometimes, but i once saw another ten that had it a lot harder and even while on throttle.

Its not a popping sound, more like scrathing (hard to describe)

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Derbi Senda DRD Sm "04 (black) => BMW F650GS "04 (yellow) => Yamaha xt660z T�n�r� "09 (blue)