car is a D2, manufactured late 97, registered 98, 4.2 quattro sport.

I wasn't sure if it could be something like the torque convertor.

My mechanic has VAG-COM so I'll ask if its reporting any error codes.

We tried eliminiating power steerring by going lock to lock and it made no difference, tried in P, D and R - again no difference. Also switched in and out of LPG

Like I said, perhaps its just me being paranoid but you know how it is with a new ( to me) car, my old Volvo I knew all its noises etc and would pick up immediately if something 'new' appeared, at the minute everything is a new noise on this. I must say though its so quiet inside I only noticed it outside when I got out whilst the engine is running to move some glass off the road in front of the car. From inside I can best describe it as sounding like when you have a plug that isn't suppressed properly and you get that whirring through the radio. Its only very quiet inside such that even at very low volume the radio drowns it out.

I'll try and get a recording of it and post up