Porsche sunroofs have always rattled.  Sad that even the 991 roof rattles but then again there is more to rattle.  Unless it is related to cold weather I would have the dealer address the problem while you have a warranty.  In the long run a rattle will result in a lose bolt or nut and then, generally one week after the warranty, the entire roof mechanism will no longer work - most likely stuck open in a rain storm.  Ask me how I know this!!  A rattling sunroof WILL fail sooner or later.