Took my '97 Defender 110 down to the West Country at the weekend and cruised back at a steady 70mph up the M3 last night and about 20 miles from London I started to hear this tapping/clicking noise from below. Now I am sure its something in the drivetrain as the noise pretty much went away when I pressed the clutch, however there was still a bit of a whine. (the engine still sounds as sweet as a nut). Now weirdly when I got into the london streets the noise reduced a lot and was more noticeable in 1,3 and 5th (but then I could be going mad) ????? . There is no juddering, no more vibration than normal through the gear stick, no other tell tale signs apart from the tapping noise ? oh but the whine got more noticeable especially when I was say sitting at the lights in neutral ?? Any ideas what this could be !!

Low oil in the Gearbox / Transfer box ?
Propshaft ? However these were recently replaced by the previous owner.
Something more drastic ?

I am new to Land Rovers so any advice much appreciated.