My Audi A6 manual diesel has developed a whine after some 90,000 miles, which is road speed dependent so could come from the gearbox. In last Sunday's Telegraph HJ mentioned using STP oil in such a gearbox to help. Which oil was used, how much put in and how?
Could you give the year and also (importantly) the number of forward speeds.
There was a problem with some 6 speed units which I can throw some light on, although there is no easy fix. Whatever you do, don't feed this transmission on snake oil.