Now we're convinced that the sound is caused by vibration as it's only happening when the revs are sitting between 1100 and 1900 RPM. Its what the service guy said it could be.
We will relay this to the service centre on Monday and I hope they find and fix the cause.
Looking forward to getting this sorted out, next adventure on the agenda for the 4motion is a good detail, I've been reading up on the CCP site and I'm really keen to get the van looking sharp. I thought I knew how to wash a car until I started reading up on it...
Tint is also on the agenda. The side shades have been doing wonders but the Multivan would look so much nicer with a dark tint.

---------- Post added 07-11-2011 at 08:00 PM ---------- Previous post was 05-11-2011 at 08:02 PM ----------

So the Multivan went in to the service centre today and came away with a replaced boost pressure valve. Apparently it was causing excessive vibration and transferring the noise through the firewall and into the cabin.
The noise is now gone so I'm happy. Shame a part needed to be replaced already and it's only done 20k. I was wrong in my earlier post on the amount of km's we'd done. I was shocked to see 20k on the clock but then it's our only car so we drive it around a fair bit, we've had it since end of Jan.

They also carried out a 23H8 service campaign. I didn't pick up the car and they didn't tell my wife what is was, she saw it on the paper when she got home. They'd closed by the time I got home so call them tomorrow and ask them what this is.
Can't see anything out there on the net about it. Does anyone know what this is?