IF under warranty, a Kymco dealer is best as they will feed back to the Kymco tech support in your country, this is often a Taiwanese guy in your country's Kymco head office.
Alternately, something I have done a few times now, I take an angle grinder and gently sand the disk brake - and I mean gently - to change the profile of the disk brake surface (remember both sides).
Did this on the brake when my unit came out of warranty when I had to buy a replacement and since then have only swapped the unit.
As someone else mentioned, the sound is caused by the pad being loose in it's holder when unused. There are leaf springs (look like metal with a few dents in it) designed to eliminate this.