Does the whiring change when you change gear on the main box or does it slowly change with road speed?
If so that'll be your transfer box on it's way out. Mine gave up the gost during my summer expedition to Morocco. ended up swapping it out in Malagar on the way home. There were bits of gear tooth stuck to the magnetic drain plug. I hauled the failed unit home but haven't stripped it down yet to see what is amiss.
Probably not what you wanted to hear
You could take the plate offf of the underside of the transfer box after draining the oil and see what's up.
Bearings aren't too costly, chewed gears and casings are a replacement box job.
The noise on my failing transfer box initially started when the box was hot and disappeared when the clutch was depressed. but towards the end was there all the time from cold. Depressing the clutch removes much of the load and the failing bearings have less to do. It could also made to be quiet when you hit the spot when the engine wasn't pulling or decelerating.