Phew, I was beginning to think my mind was playing tricks. The mark 6 I had never had the waste gate rattle, so when I first heard this metallic noise I was concerned, but now after reading this post I am feeling more relaxed. One solution was to leave in race mode which drowns out the metallic noise
I only really noticed it when the soundaktor got turned down! It does seem characteristic for the car, but "rattle" seems like a misnomer.
Update: Just been out in the car this morning and went into normal mmode so I could hear the issue better - the "rattle" is definitely quieter after a couple of miles when the water is warm, and is heard when you're gently pressing on from 1300 revs to about 2500 revs, and it is more of a hollow ringing - like a muffled bicycle bell. The fact it gets quieter on warm-up reassures me.
The car is booked in for next Friday afternoon for the ACC turning itself off every once in a while. They suggested taking it out for a test drive to replicate the fault or keeping it a few days to drive it about, after i'd said 5 out of 6 times it has happened on a long sweeping bend in exiting the Tyne Tunnel north-bound.
I shot that suggestion down straight away - they want to keep my car until an issue that manifests itself an average of once per week recurs, using my fuel. They probably want to go through the Tyne tunnel multiple times at £3 per return journey on my pre-paid permit too.
The car has an array of fault logging sensors to interrogate - they can start there rather than someone having a weeks jaunt in my car, using my fuel.