Did you find out what this issue was? I have Red edition Arona 21 plate. Had it since august and I now have this rattling noise like something is loose just like you said passenger side footwell. I find it is more prominent when in 3rd gear.
Assuming it is nothing obvious like something in door pocket or glovebox, (and there have been cases of locking wheel nut adapters being put loose in the glovebox) then my hunch is there is something that has fallen and got stuck at the top of the carpet / mat and is vibrating.
Another common noise is the CV joint in the drive shaft (which is under a bellows like rubber boot) although this tends to be more of a squeak. Just spray some silicone lubricant on it (available from most hardware or DIY shops).
If it is only rattling in one gear, then I would be getting dealer to look at it under warranty, as suggests a piece of metal is broken off a cog and loose in the gearbox