Thank you for your reply Steve (and Western).
It's only done 19,000 miles. You could indeed be right with the transfer box, I will check the tourqe on it tomorrow though just to be sure and clean it off.
The diff however was wet with oil, ie. you could wipe it off with your finger. I will check the level of this tomorrow, and do as you say
The noise only happens first thing in the morning really (or when cold) and is there when the clutch is dipped, engine at idle, and in gear... but only after I've moved her (ie. used the clutch to produce some drive). I find it hard to re-produce the noise afterwards. I've not tried revving the engine whilst the noise is there, maybe this will help pinpoint what it is?
Again thanks for your reply!