My Disco Td5 rattles under load in 5th. Help!

Just recently my 2000 Disco with a manual box rattles, (or sounds like something's bouncing off the road at least) in 5th under load. No other gears just 5th. It doesn't really get any louder just speeds up. It happened two weeks ago then stopped and started again today. It doesn't sound too 'metalically' but does have a slight ring to it. If I put my head down near the centre console it becomes more evident. I'm new to Landy Zone but has helped me out in the past from other posts so it's time I joined and could help some other poor soul along with hopefully someone helping me too.

Could be rear support bearings in gearbox,common for them to wear,look on Ashcroft website r380 rear support bearings ,best to repair it before it damages gears aswell,had to do my 300tdi ones as had a little play in them,