A weird sound of Runner ST 200 (2012)

Hello

I have a gilera runner st 200 -2012

working this scooter for 7700 kms.

Somehow, It makes me angry, whenever it accelerates from 20 to 40 very slowly, it makes a huge sound like "gırrrr"

 

I think there is a problem in variator? what do you think? which part should I need to replace?

 

thanks

seckin