I think there could be a couple of things here and an easy way to check them.
Cooling Fan
The cooling fan in the Navara has a clutch that should release when the engine is turned off. The engine may stop and the fan might still spin over for a few seconds. If there's a bearing issue in there, you'll hear it. The fan isn't dirt cheap, but you won't have to sell your left one to replace it.
While the engine is turned off, open the bonnet and just grab the fan and give it a spin. It should resist your efforts slightly but the rotation should be very, very smooth without any grittiness to the motion. If it feels rough, this is probably the problem.
Turbo Bearings
These are more expensive, but you need to take care not to make it any worse. First, clean the outside of the air intake lines around the turbo charger inlet. You do not want to risk even a speck of dirt or sand getting in. Undo the clamp holding the hose onto the turbo charger then thoroughly ensure your hands are clean and have nothing under your fingernails. Remove the hose and give the turbocharger a spin. Try to wobble the shaft.
The turbo should turn over really, really easily and the shaft should not wobble at all. If it feels rough to turn, there's probably a buildup of varnish in the turbo bearings caused by shutting the motor down before the turbo has stopped spinning and cooled enough.
If the shaft wobbles, the varnish has been in there long enough to destroy the bearings. You need to determine if the compressor has ever touched the sides of the housing - it's a dismantle job, unfortunately, but a competent workshop should be able to do that and tell you if it's just a bearing replacement (and varnish removal) that you need, or if you need to replace the whole turbo.
I don't think it's the air conditioning compressor, because they don't use clutches in the compressor belt drives any more - they just vary the compressor usage from 0% to 100%.
You could try giving the alternator shaft a good shake with your hand. It should not wobble or push in or out at all - but I'm not sure there's enough momentum in the alternator for it to run for 6 seconds after a shutdown.