[Seatman]

Hi there, I have heard of this noise before but I can't remember where offhand. You're right though with the actuator and N75, the other thing that can cause the overboost is the turbo vanes or waste gate being sticky inside due to carbon build up from the exhaust and possibly a damaged vacuum pipe. Have a look here if you haven't already.

http://www.myturbodiesel.com/1000q/multi/low-power-fix-TDI.htm

If I find the thing about the strange noise, I'll get back to you.