Hi I'm a newbie and I have a problem with my Jimny, please help

Hi I have an 04 Jimny Mode, I have recently bought it, I get a groaning noise from time to time, some times it will go a week with no problems and then I will get it three times in a day, it doesn't affect the driving, if I speed up it fades out, if I slow down it will get worse and as I come to a stop there will be a clunk and then it's fine, I'm pretty mechanically minded but this is baffling me.

Could be any of a number of things. Is the car modified at all? If you speed up does it stop and not come back or just fade and reappear once slowing?

If I speed right up it disappears, if I just go a little faster it fades and then comes straight back again, it's very noisy when it does usually happens around 50mph, if I then go to a lower speed it becomes horrendous, but if I go to about 60 it stops, if come to a complete stop I get a clunk as if a gear is engaging.
Then I can go a whole week with nothing, having said that I'm usually only doing short journeys and no more than 45mph.
The car has not been modified, there is an oil leak on the transfer case

Manual or auto?

Manual

Hmm. It could be that the transfer box chain is slipping but i wouldn't expect you to still have drive if it were. It hasn't been used on tarmacadam in 4x4 to your knowledge has it? It still steers ok? Is there any significant movement of either end of the rear propshaft ie out of the transfer or into the diff? With the rear wheels in the air on axle stands does everything feel free and smooth to rotate? If the propshaft is held do the rear wheels contrarotate? With one wheel held and the other rotated does the propshaft spin free? With the car back on the floor and on a soft or loose surface does 4x4 engage and disengage smoothly? When it's making the noise can you feel it in the steering or gear levers? The exhaust system is correctly positioned and securely affixed? There's nothing wrapped around the propshaft?

Sorry it's more questions than answers at the moment but we need to try and isolate what is happening where.