From an internet explanation sounds more like the gear bag extension / 5th gear bearing.
as opposed to the transfer...
If the tcase is in nuetral and the gbag is in 5th and your hearing noises me be thinking its gbag or perhaps input transfer case and PTO bearing?  Its easy to visual the r380 output shaft and input gear by pulling the PTO... 
The extension housing of the R380; 5th gear, bearing shaft and oil pump are all known to play up?
Steve
				
			 
			
		 
			
				
			
			
				'95 130 dual cab fender (gone to a better universe)
'10 130 dual cab fender (getting to know it's neurons)
			
			
		 
	
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