Grinding sounds from the t box when shifting low high

[Ace of heart]

Hi all. I have a 2010  lc prado 150 while shifting to 4 low or back, the t box makes grinding sounds. 

It doesn't alwsys happen and the 4 low works well. 

My mechanic doesn't know where to start. 

Anyone can shade a light on this issue? 

[Cyker]

As someone totally unqualified to give any advice, my first response would be check fluid levels and search for fluid leaks for the transfer box. If the clutch has any role in that, would be worth checking that too but if normal gear shifting is fine that's probably okay.