There are loads of rattles and noise in the mk5 cabin. I have fixed about 5 or 6 of them, but new ones keep appearing. That's the donside of VW's cost cutting. The plastics are all cheap hard plastics and thus you get rattles. The mk4 was constructed with much higher quality materials... wasn't as pretty though.
The best solution I have found is to track down the offending rattle, remove the plastic trim and line the rear with felt. Anywhere that plastic touches plastic, put felt in between. This works brilliantly.