Just got a reply on the flywheel so this is all good. However, the mystery where the grinding noise comes from still remains.
Someone on another forum told me that he had a similar grinding noise and that he just removed the variator and then assembled the flywheel and fan first and last assembled the variator. He wrote something about that this shifted the crankshaft slightly which stopped his scooter from sounding so nasty.
Sounds like a very strange solution to me, what do you guys think?