Land Rover Repair: Noise comming from Transmission or TF box, viscous coupler, cement mixer

Question
I own a 91 Range Rover Classic and have a problem. Can you help, at idle when the car warms up I get this rumbling noise comming from under the car. I have read that the chain in the Transfer Box can stretch and make a simular sound but this would not be going around at idle would it. It really does sound like someone has put stones in a cement mixer? Any Ideas? The car driver great and shifts really well and smooth.
Also what is the best way to check that the transfer box is locking up correctly, as I had problems wheeling the other week and my friend said he could not see the drive shaft going round when the back wheels where spining.

Answer
The noise under the car may be rattles from a failed catalytic converter.  You should be able to diagnose a noise like that pretty easily by putting the vehicle on a lift while it's running and listening and looking.

The way you test the transfer case is to make tight circles in a paved parking lot.  If the coupler in the transfer case is binding you will actually feel the wheels chattering and skidding through the tight turns.  If the truck rolls smoothly through the turn the viscous coupler is fine.

The driveshafts to both front and rear wheels will spin whenever the car moves.