strange noise while in 4wd

while in 4wd and especially in low low going down hill, there is a noise that almost sounds like a rubbing sound, but only when going down bumps. i have checked all over, and there are not rubbing marks on the guards etc. a mate told me that in the transfer case of the Nissan Patrol, (2006) there is a chain...could it be this chain making that noise?
PS...the motor is a 6.5 Chev and the gearbox is the original Nissan.
John