Transmission noise 4th and 5th gear..

Suddenly somewhere amongst the drivetrain of my defender it has developed a particulary unpleasant rattle/resonating vibration which seems to be coming from underneath the seatbox area. Happens in fourth and fifth gear when under load. Just parked it up put the transfer in nuetral and 1 two and three are quiet working through the gears but 4th and 5th when selected are definately knocking a little.... very frustrated as this is my daily driver at the moment. Im not really certain if both noises are related. Any advice much appreciated.

Check your oil levels in the main and transfer boxes.
Have you any leaks?
Check the UJ's for play and grease them up whilst you are down there.
Check for movement on the handbrake drum.
The above might not seem applicable, but if you are getting noise/vibration in 4th and 5th, it is possible thay they might be 'speed-related'.

^ thanks. Transfer box oil level ok. Props recently off and regreased - no play or binding though i know how fickle these can be have just ordered new uj’s. Just took it out again and it does seem speed related. Going from 2nd straight into 4th to really load up the box at low speed it doesnt happen. Seems to be 30mph and above.

Check / change oil in main box. Ashcroft site has info about common R380 problems.

I would also be inclined to check your diff and swivel housing levels as it does apppear to be speed-related.
A slight 'knock' on take-up is not particularly uncommon with Landies.

Ok got home from work and got underneath with the intent of running through the afforementioned checks and whilst checking for play on the front prop noticed that it wasnt phased!!! It came apart on me whilst i was rebuilding the vehicle a few month ago and i must of got it confused with the rear..... so 4 nuts and bolts later i phased it as close to 30 degree as possible ( going off the diagram in the wsm) took her out and i was not able to replicate the noise. Im really hesitant and nervous here of proclaiming this a success , maybe it was just an anomoly of sorts that the noise did not reappear , and it will be back with a vengeance again tommorow. but for now im keeping my fingers crossed.

So it was the one thing that wasn't mentioned!
To be honest, I never thought about phasing, as not many people pull the shafts apart.
Pleased to hear that it seems to have sorted it.