300tdi diff or transfer box whine/drone

Hi,
I've recently purchased my first proper landrover a 1994 300tdi but i have a problem, well more than one but her is the first, as you get above 25 to 30 mph there is a drone/whine which increases with speed but does not change if you are in a different gear or back off it just changes with the speed, gets louder to near deafening point at motorway speeds. my assumption (ive been told never assume but) is that it is a bearing, but i cannot establish whether it is in the transfer box or the front diff. is their an easy diy way of finding where the fault is without taking it all apart and looking at each bearing.
i have checked for any play on the drive shaft output near the parking brake and into the diff but cannot feel any.
any suggestions would be gratefully received.
Tony

try with one prop off at a time in diff lock ,will help locate front back or t/box

Try putting yeh clutch in whilst driving!

I'd put me money on the B/T !

when did it last have a full oil change, I mean the lot, G/B T/B diffs, engine.
if you don't know, try that first, wont do any harm, hours work, £30

Thanks,
I'll try the prop shaft thing at the weekend.
I've tried pressing the clutch, it makes no difference to the noise whether the drive is under load or not, i think i will have to get the wheels off the ground and drive the rear without the front shaft on, then the front without the rear etc.
Thanks again

oils done a few weeks ago.