viscous coupling ???

we have a rubbing noise , sounds like a dry bearing , when we drive the front and rear drive shaft turn , how does the vc work as i was told that the rear wheels only get driven when they slip and the vc is activated , does the rear drive shaft turn all the time ? or have i got a problem causing this sound or is it just one of the bearings perhaps ???? thanks Steph....

It's permanent 4x4. The VC slips to compensate for the gear reduction of the IRD. Noise from between the seats is commonly from the VCU support bearings.