My car has now done 85k miles and I am now aware of a faint whine (sounds like transmission) when cruising at approx 75 mph but only when the car has heated up fully after say - 20 mins or so of driving. The noise comes in at around 75 mph and goes away at around 80 mph. It is only noticeable because everything else in the car is so quiet - does anyone know what this is?

Transfer Box? When were the oil levels last checked and/or changed?

I wouldn't ignore it pal that's for sure , I agree with the above , having the oil level in the box checked and if it hasn't had a service at all then poss get that done

I believe the transmission oils were changed at a previous extended service last year (it looks like I paid for this) but to be sure, I will do the job again (myself) and then I can be certain.

Thanks for your suggestions.

Regards