Hi All,
I promised I would get back once I had the "whistling" problem rectified so here goes:
After one service, where the dealership failed to rectify the problem, I returned and they contacted VW in Germany to ask what could be done. VW determined that they wanted additional testing done and for the fault to be diagnosed. This was done and it turns out the problem is related to the rain channel (black rubber strip) that sits along the vertical edges of the windscreen. The technician explained that air can enter the grill, travel under the bonnet, then depending how the rain channel sits -emit a whistle much like we "whistle" once we get the right shape with our lips etc;
Anyways, some adjustments were made around the grill, I'm not sure where exactly, but I understand there are a number of adjustable components under there, and a new rain channel was fitted to my GTI.
I picked her up last night and can happily report that it is now whisper quiet and cabin noise is markedly less. The whisle has disappeared as well.
The lesson is - if you are experiencing this issue, ask your technician to look at the rain channels and let them know that VW is aware of the fault and there is a known fix.
cheers,
Ozzyc