I have a 1998 a4 2.5 tdi avant (6 speed manual) and have noticed a sort of whine from the gearbox. Only really apparent in 5th and 6th. It does it both while accelarating and deccelarating, although the tone changes. If I then throttle on to get neutral revs (neither accelarating nor deccelarating) with my speed the noise stops.
Could it be just a gearbox getting worn? 157K miles. Or needs oil? How do I check the oil level?