Actully sinthetic oil can make them run slightly quieter, its not going to make a noisy box silent though. Useing it purely for acustic resions will probubly disapoint, its more of a slight side affect that you treat as a bonus.
I'm sure my 200Tdi got slightly quieter on 10W40 semi synthetic than it was on whatever it had in it before, but that was probubly just because it needed a service and the new oil had less
**** in it.