to add to this issue, my C600 still has'nt made this noise and i still havn't felt any of the usual belt slip issues i have had with many of the auto's i have owned in all the years of riding. my little vespa does exactly what you guy's are saying and i know that the belt is of an unknown milage and probably needs replacing.
to me the issue is just belt slipage on the pulleys and this is normal, i use brake cleaner to clean the dust off the surfaces which does help but probably a new belt is the best solution if you can't live with it but that would get pricey.
on my C600, i still have had no concern to even look in on the issue as it still is ok. my cover has never been off. i know that the 5000 miles isn't a lot and tbh, i would not expect to have to look in at that milage. i do think a lot of this issue could be down to usage and particularly heat effect.
4 years ago i owned a piaggio 125 typhoon and the bark on that was amazing, every time i set off it went off like a rocket and that was almost the noise it made, the noise was between a firework going off and a turbo dump valve. my friend had an aprilia SR125 with the same motor and this made exactly the same noise. i kept on top of cleaning the pulleys and belt but it made no difference.
the only bug bear i can think of with this is every now and then, combined with this noise is that you get a vibe through the scoot when the belt slips but i found this to be not every time and again, this was due to usage.
i think that a lot of town work highlights this, the fact that the belt is up and down the ramps of the variator and slipping due to many take off's.
my C600 really only gets used hard and fast on either long motorway runs or blasting around the country lanes and i am sure this is why i still hav'nt yet come accross mine having this issue.
i do hope this kinda makes you guys feel a little more at ease with your issues, i wish i could shed more light on this but i hav'nt experienced this on any of the C600/650'S i have worked on but do know other auto's have this issue.