In days past a knock that happens only when turning would make me think of a worn outer CV joint. This used to be very common on the BL transverse engine cars (old mini, ADO16 etc), but you don't get these failing much nowadays. The last one I've had to change was on my old Renault 5 (and that was after at least 150,000 miles). The giveaway is that the knock frequency is speed dependent; sometimes you can feel the play in the joint if you hold the driveshaft firmly and try to wiggle it about.
On a 500, my first thought would still be suspension (because it's known to be weak and lots of folks report problems with it), but I wouldn't rule out the CV joint. Check the gaiters first; if one is split, then this becomes much more likely.
