Just wondering if you looked at the small gear on the starter? If the teeth are worn badly, that could cause a grinding noise. If you hear a winding or singing sound upon deceleration, especially at high speed, this is the final reduction gears in the gear box and even with the proper amount of gear lube, this is normal. I have often wondered why Kymco calls for only 7.1 oz. of gear lube in the gearbox on my DT300, when every other brand of scooter is , fill to overflow?? Without actually hearing a noise or sound, it is difficult to pinpoint where it's coming from or what causes it. I wish you luck with this and welcome to the jungle, er or forum.
Mac