Audi A3 Cabriolet 2015 window loose?

Hi

I have an Audi A3 Cabriolet 2015 and with the roof and windows up when I close the doors I get a satisfying thump, but when the roof and windows are down when I shut the door, I get a rattle as though the glass is loose in its frame. I have asked at the Audi dealer and they seem to be a bit vague as to whether it is a problem or not, I did check a car that they had for sale and that seemed to have a solid feel and sound to it when the roof and windows were down and the door closed. Its not such a problem when driving, only when I shut the door.

The dealer doesn't seem to be able to tell me if its as designed, or if there is any adjustment available, simply that if I were to book it in it would take them at least an hour to look for a problem. 

Does anyone have any experience of the window rattling in its frame with the roof and widow down when the doors are shut, or have knowledge of the door construction and would know if there is something that can be tighten up? When I looked at a drawing of the window assembly in a workshop manual, taking the inner panel off seems to leave a metal panel that the glass slides behind so it doesn't look like the frame the glass slides into is going to be easily accessible even with the trim removed. 

Any insight would be appreciated.

M