d3

the old man has just come back from france and his d3 has developed a rumble like a wheel bearing on it's way out. Any ideas as he checked wheel bearings they seem ok. All the drive train looks ok. After speaking to him he hasn't done diff oil change so told him to change oil in them plus the gearbox is sealed but what about the transfer box on the d3?
p.s. he is coming up to retirement and spending enough time under my feet so need answers so it will keep him busy

cheers for you replies its ok found a great thread on another site, must say it is very professional with loads of info but it was like, a garden party with the queen compared with the loonies on her god i love this site