I have owned the above car for about three and a half years and done about 40,000 kms in that time. Over the last year or more I have become aware of a clunking sound on overrun when my foot is taken off the accelerator in light load situations at about 40kms per hour, particularly on very smooth roads.

Upon inspection I have discovered that there is what appears to be excessive backlash between the crown wheel and pinion in the differential, which I think is causing the clunking sound. I have come to this conclusion as all the components of the driveshaft, axles and rear suspension seem to be in good order with nothing loose. Here is a video to backlash (with sound) https://photos.app.goo.gl/tTatCiTYt7zK5HZ89 It should be noted that there is no oil leaks at the diff and no diff whine . The oil in the diff is clean and at the correct level. There was a very small amount of black sludge attached to the magnet on the level bung.

Researching of 2009 XF Jaguars online brings to light that the failure of these diffs is a known problem with some failing in the first few years of their life. It is thought the cause was related to the diff housing not being cleaned out thoroughly after manufacture. I understand this issue was covered under warranty.

1. Does anyone know what the specified oil is for these diffs?
2. Do you think the noise I am hearing could relate to excessive crown wheel and pinion backlash?
3. Is diff back lash noise likely to be reduced or eliminated with a thicker grade oil or an oil additive?

Any other suggestions appreciated.