First, make sure the gear ratios are the same. If it has auto locking hubs, and the ratios are different, it will force engagement, which could sound like you describe. The front diff should not be engaging unless the transfer case is engaged in 4x4. If one (or both) of the auto locking hubs is damaged or frozen (engaged), it will cause the other to engage.

Otherwise, look for something broken. Anything making a loud noise should show some indication of something wrong. Start with a full service. Remove and repack bearings, pull the cover and inspect the R/P and carrier bearings, etc. If it continues, remove the shafts and check the floater bearings and U-Joints (knuckles). In any case, a disengaged front diff should only be turning on the wheel bearings, with idle shafts.