Allroad 3.5 TDI - noises when slowing down and turning

[Amphitryon]

I have a 2.5TDI (AKE) automatic allroad C5 which has started making some concerning noises.

The first is a kind of twittering noise when taking my foot off the accelerator, especially noticeable when going down hill, i.e. when engine braking. Gently pressing the accelerator causes the noise to stop, as does moving the transmission into 'N'.

The second is a clicking, especially while cornering, more when turning left than right. Accelerating out of a roundabout seems to be a good trigger for this. Sometimes, after straightening up, this takes a few seconds to stop. This one sounds like CV joint. So far, I have replaced the right hand outer CV joint but the noise remains. None of them click when jacking the car up and turning the wheels by hand and none of them feel to have any play in them. There are eight in total so I don't want to be just working through them all.

Anyone had this or know of any other possibilities? If this is a CV joint, how do I tell which one? From inside the car the noise appears to come from whichever window I have open rather than from any part of the car in particular.

[Amphitryon]

P.S. on changing CV joints, on offside (UK vehicle, right hand side) I was able to change the outer CV joint by removing the bolt, turning the steering full lock left, and just withdrawing the splined shaft on the end of the joint from the hub, then removing the joint from the end of the driveshaft. It was not necessary to dismantle any of the suspension and there seemed quite a bit of in/out movement at the inner CV joint to accommodate that - I assume that feature of the inner joint it to allow for the effective length of the driveshaft to vary as the suspension moves up and down.

On the nearside, it is barely possible to disengage the splines from the centre of the hub and there is certainly not enough free movement to get the joint completely free of the hub. The difference seems to be that there is a little less in/out movement in the inner joint.

Is this an expected difference? Is there some asymmetry in the drive train that is not immediately obvious? I know on cars with front wheel drive and a transverse engine the final drive is usually over one side so one driveshaft is longer than the other but this car has a longitudinal engine. Or does the difference indicate a fault in one of the inner joints?

[Crasher]

I would expect the twittering noise to be the turbo waste-gate starting to stick and CV’s can be very difficult to diagnose especially as lifting the car up makes them pick up grease from where they normally run. When was the front differential oil last changed?

[Amphitryon]

...When was the front differential oil last changed?

Thanks, Chrasher. I have never done it and the car has done 133,000 - is it on the service schedule? Is there a diagram that shows where the various transmission pieces are? I didn't realise there was a separate front diff from the (auto) transmission. I was looking what I could see without needing to jack the car up and crawl under and found this:

IMG_1993.jpg
This is take from the drivers side so the front of the vehicle is to the right of the picture. Which is the part blackened from oil on the outside? Is this the centre diff? It looks like there is a filler plug part way up which is probably the source of the oil. Does this mean someone has just over-filled it in the past or should I be worried that this is running low on oil?

[Crasher]

The box has three separate sections contains their own fluids, the front diff, gear set and centre diff. The front diff on almost all boxes can start to hum and vibrate as the oil ages loosing the shear properties of the long chain molecules… basically it wears out.