Hi
I can't talk from experience, but they do seem to suffer from water leaks on the rear doors, so the rattling may be down to incorrect door adjustment.
The other problem is that most courtsey/demonstrator vans are used a spare part donors so there was probably no spare wheel in the frame under the rear cargo floor. That makes a terrible racket when empty as there is nothing to secure it from rattling around. As most dealer principals ban this practice of robbing parts to get a customer's car going, it is usually done on the 'QT' and nobody knows. Unfortunately, the part replacement and fitting are not often very well policed so I would think very carefully about buying an ex - demonstrator.
Many years ago my daughter brought a preregistered Fiat Punto that had been in stock for a few months. When it arrived, most of the aircleaner securing mountings and rubbers were missing, as was the rubber gasket that sealed the airfilter to the fuel injection system.
Fiat boast that the new van is very quiet to drive, so who knows what the problem is?
If you are thinking of buying one in the future, take a test drive in one then, and also try the Citreon and Peugeot and see if they are the same. Personally I think the latest models look awful so I probably wouldn't get to do a test drive in the first place.
Spruce.