Since it appears to only do it under load it could well be wear on the gear teeth. Though that would probably start with little noise and get louder as the wrear increased. Was there any water in the gear oil when you drained it? It could also be, as mentioned, something not even in your gearbox, but I wouldn't expect that to only appear under load. Have you tried putting the transfer box in neutral and running it in 3rd and 4th to see if you can still hear it?