Grinding noise from transfer case/front driveshaft

[raisemhigh]

With truck in 4x4 w/ hubs locked (warn manual locking hubs), I am getting a grinding sound coming from my transfer case/front drive shaft while driving. It only happens when I am at part throttle (slightly on the gas). If I am full gas, or completely off the gas it doesnt make the sound.

Also is my front drive shaft suppose to turn freely, when the hubs are not locked and not in 4x4

Any ideas???