Tejou,
Not sure if you've checked, but could be the nut being loose on the balance shaft allowing the gear to move back and forward on the key, making a loud banging noise, as I learned not so long ago. Caused by lugging the bike at low revs and the big single cylinder acting like an impact driver undoing the nut I seem to think. Doesnt mean you have to replace the balance shaft or take the engine apart completely, but best to replace the key on the shaft though and torque it up good.
Hope this helps
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