Whining noise Nissan Patrol 1990 GQ 4.2L Petrol Manual.
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I take it from point 1 that you changed the engine oil?
The noise you describe is coming from the gearbox. The reason you can't hear the noise in 4th is because that is the only gear that does not actually use the gears in the gearbox, it's direct drive-input and output shafts locked together. It is most likely caused by either worn gears or bearings (or both). I can only guess that you are hearing the noise now because the engine is running a bit quieter after the oil change.
Check the gearbox oil by removing the fill plug in the side of the gearbox. It's about half way up one side (I can't remember which side) of the gearbox casing. The oil should be level with the bottom of the fill plug hole. Check the transfer case oil level the same way while you are under there.
Replace your battery ASAP. It won't do the alternator any good constantly trying to charge a dead battery. An alternator that is working hard will make a whining sound that can be heard easily in the engine bay with the engine at idle or just above. Will be a bit harder to hear with diesels.
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