Gear/transfer box whine (R380)

Well first off my disco passed its mot on fri after nearly a years worth of work getting done to it but all is not well.

Once I got out on the road to head to the mot station I could hear a whine coming from ether the transfer or gear box, now it only happens when in gear and sounds a lot worse when it's in the overrun.

So first thing I did was change the gear oil but that did not sort it. Will try the transfer box oil tonight but not holding out for any change

Now I have had a search both on here and good old google to find out how to find out for sure if it's the gear box or transfer box but could not find a answer.

Is there anyone here that could tell me or advise me on how to go about working out what component is at fault.

Also it's a strange one since the box was as quiet as they could be last year before it was taken off the road..

When I had a similar issue with my D2 I spoke to Dave Ashcroft who said there was no reliable method to pin it down to either the gearbox of transfer box.

I had a spare R380 available so swapped that in only to have the whine continue.

So I bought a used tbox and swapped that in which cured the problem.

If I had to do it again I'd be inclined to do the tbox first, being easier to get to and take off...