Last of the older model, you maybe seen my post. Jag wanter over £1200+vat to replace my Diff Pinions Seal last month. Instead I bough the oil and seal from them, they even gave me 20% trade discount and a friend who is an AA mechanic (also has a wheel free lift in his garage) he did it in three hours and asked for £60. I gave him £150.
Now on the noise, if it is from the diff I would assume it's junked already. In my case it was paranoia once I was aware I had a leaking diff, I was hearing all sorts (not saying you are paranoid), thing is, the diff only holds one litre and it is like a fully synthetic oil, not like the old thick stuff. When he dropped my oil there was 850ml, so only lost 150ml over time but the casing was very wet and occasionally there would be a mark on my mono blocks.
I would advise finding a back street garage with an older mechanic that will take on the job, and get the seal changed. Worth £300 say, to work out if it's junk which I would be surprised if it was. Jag would have quoted for a diff not and seal change if they felt the diff was junked surely???? Anyway good luck, I feel your pain.