It's good to hear that certified Kymco mechanics have checked this out, and could find nothing amiss.
I think it's a noise that is inherent to the Kymco engine, I call it graveling, and it is noted in my 500 X, my 250 X, and even my Grand Vista.
When I get some more miles on one of the scoots, I'll try a Malossi variator, and I even have a J Costa for the 500, but at present, there is nothing wrong with any of these machines, so I'm not going to pick them apart and re-engineer fully functioning scooters.
That's what I have Chinese scooters for, research and development, and performance tinkering.
I've already dicked around with the Kymco carbs and exhaust, and while it was fun, it didn't produce any grand improvement, just a better sound and a little more pep.
I did try some 18 gram Dr, Pulley sliders in the 250X, but the RPMs were too high, so I went back to stock rollers. The tinklinking sound that had disconcerted me at first seems to have abated over time, if it returns, or I get bored, I'll open up the CVT again.
Kymcos are awesome, so don't try to change too much as long as it's just a funny noise.