Droning noise from D4 4.2tdi

[velour]

Hi all,

Have a D4 4.2tdi that I bought 2 1/2 years ago with 27k. It's now got 57k miles.

Yesterday it started making a noticeable droning / humming noise, difficult to tell where it comes from but gets worse with road speed. It's worst at about 70 going down to 50 with engine braking.

On a trip to Southampton today the sports differential warning light (yellow) came on but has now gone again. Took it to a dealer who cleared the warning, took it for a test drive and said they could not find anything wrong. Noise is still there though. Definitely not tyres as new ones fitted very recently.

Any thoughts on how to proceed - just brace myself and go back to the dealer to investigate the noise? Is it likely to be anything other than the diff?

Cheers