Thanks Frank,
Yes, some very practical suggestions here. Pretty sure it (the noise) is originating somewhere in the gearbox as the symptoms are evident with the vehicle stationary and with transfer case in neutral and gearbox spinning in either 4th or 5th. Rumble noise is worse at idle but it is only evident when vehicle is in either 4th or 5th gear at idle speed.
When the engine is gently sped up, to say 2000 rpm in either 4th or 5th gear, the "rumble" then tends to a rough "Zizzy Whine". No similar symptoms with box in 1st, 2nd, 3rd, reverse or neutral. Honestly can't confirm if the rumble has been there for a long period as only started close examination when unusual noise started to appear while driving in 4th and 5th gear. Its a strange one for sure.
Didn't mention earlier, but had dropped the transmission oil as the logical start to the investigation a day before writing the first post . (Drain was performed as per your advices i.e. oil hot and dropped pretty much immediately after stopping).
The oil in the box had been in there for around 18 months (10,000km) and the vehicle has had quite a few tough days out over the period. Its not routinely a "daily drive", mostly used for 4WD holidays and dirty weekends only.
The transmission sump plug magnet had nothing of note on it when wiped on a white tissue (see attached photo as tissue was still in the garage bin).........minor dark slurry and just a few very tiny metal flakes. Pretty typical deposits I would have thought, all things considered? The drained oil is still sitting in a clean container and I will strain through a fine cloth tonight to see if there was anything of note suspended in it.
Re the Nulon G70 fiction additive mentioned. (
https://www.nulon.com.au/products/Specialty_Products/Manual_Gearbox_and_Diff_Treatment).
Have used the product over many years with no problems and can honestly say that it does have a very noticeable and positive effect on rough-shifting. It does however turn the oil a bit "milky looking" because of its creamy texture so can be a little surprising when someone sees the oil draining out and its not the usual honey or clear colour that was put in. Only ever read positive comments and have never heard of anyone associating the G70 additive with any problems (has been in the market for probably close on 20 years now I'd think?). Others on the forum may also have experience with the product?
In the interim, guess I'll just need to keep on with the investigations............
Thanks again. Input is much appreciated. Regards Geoff.