Cracking / knocking noise coming up thru 4x4 lever

[McSpud]

Hey Guys,
While using the '88 3CX today, noticed that, while using the front bucket to load some soil, that there is a nasty'ish knocking or clanging noise coming up through the 4x4 lever into the cab when in the 4x4 position. Totally silent in 2WD.
I followed this lever to a control cable just behind the pedals, and arrives at what seems a small gearbox on the rhs of the bell housing where the front drive shaft attaches.
Do any of you know what could be causing this noise, it only happens when the machine is put under pressure, but it doesnt sound nice at all.
Hopefully someone can help, dont want to open up anything unless I've some idea what I'm getting myself into...
Cheers
Seanie

[McSpud]

Tried this again to day guys, and still the same story, its like something is only half engaging inside where the 4 wheel drive cable goes in, under the cab.
its a kind of slipping / clicking noise.
Do any of you know what kind of mechanism is in there, and if its possible that this noise could be coming from there?
Thanks a mill!