I seldom have any rattle from the front vent.
On the other hand I have terrible rattle from back seat.

I cannot place it 100%, but it seems to come from the right side. I've been back there looking around many a times, found nothing loose, but still every time I drive over not so smooth pavement I have that rattle.
The seat is fully locked in upright position.
The back shelf sits secure in its housings. I tried to shake the shelf by hand, but the sound it makes does not resemble the rattle.
The sit belt clamps do not touch any plastic bits.
I don't know what could make that rattle, but it sounds like loose plastic bits making a shake'n'screech.
That rattle destroys the whole joy of driving for me. I drove 8 year old MB 500CL, and BMW M5 - no rattles whatsoever, just a feel of high build quality and robustness. In the GTI it feels like it's made of thin plastic bits ready to give way....

Any suggestions?