Whining noise in 4x4

[XPlorer4x4]

I have a 1991 Ford Explorer 4x4 and when i lock in the hubs and push the switch for the 4x4 i get a whining noise that sounds like it's coming from the transfer case. I'm not sure if it's from the transfer case though because if you leave the hubs unlocked and push the 4x4 switch there is no sound. Does anyone have any ideas of what this could be???

Thanks

 

[toad]

Only when your moving? Sounds like you already have manual hubs so thats out, so U-joints , low diff. or T-case fluid.