Final drive, diff whine.

[Bigian]

Since hitting 42000 miles my 1.6 Diesel XTR has had a whine on the diff at bang on 40mph, it clears again at 45mph. I've changed to a good quality slightly heavier gearbox oil but it hasn't improved matters, I didn't really expect it would but wanted to make sure it had enough lubrication.
I know it isn't wheel bearings or anything else rubbing, has anyone else got this issue on theirs or has anyone had the diff repaired?

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