Hi Laker,
I just changed the transfer box on my X5 because of the same problems you are having, inside the transfer box it would appear that the chain is jumping because there is no tensioner ! it would appear that there are two anti-slap pads instead of a normal tensioner and both of these had worn out allowing the chain to "Flap about" and jump . I'm trying to get these parts from a company called Magna in the USA because the it's a GM box but no luck at the moment. I brought a S/Hand gearbox with the transfer box for £1200 had I known that it was just the transfer box I could have brought a new one from BMW for £1250.
Hope this helps.
Regards
Tony