I have recently purchased an A4 and on the way home I heard the faint noise of what could be a diff whine, a trip to my local garage confirmed that the whine is most likely the diff so they replaced the gearbox oil, what they found was that the gearbox only had 750ml of oil in it, a quick call to Audi confirmed it should be 2.2 litres. The gearbox was then filled with the correct oil. Luckily the noise has quietened a good bit but is JUST audible. Anybody replaced the 6 spd box? How much would I be looking at for a replacement or rebuild should he noise get any worse?