vibration from transfer box area

hi all, i have a 1999 range rover DT which has a vibration coming from the (borg warner)transfer box area, this only happens on tick over, if you push up or pull down on the box it goes away, i've checked everything i can think of from the exhaust and gearbox and engine mounts to taking the hand brake and props off, i'm pretty stumped. can anybody throw any light on this one for me please.