Righto, been under the car and can feel or see no obvious faults with the propshaft or drive shafts, gearbox mounts seem to be fine also.
I got my mate to stand along side the car whilst I replicated the sound and he identified it as being from the area (underneath the car) behind the front wheel but forward of the wing mirror, virtually inline with the door shut which uncannily enough is inline with the transfer box....thanks Nige!
Sooooooooo, my next question would be; are these boxes interchangable between any 5 speed Awd Manual Subaru? Is it a simple case of removing old one and bolting on the other (obviously with gearbox dropped

)? Anyone got one lying around?