1990 4matic whine warning?

[mwagon]

I looked at a 1990 300TE 4matic with 165k miles thats for sale cheap, but maybe not cheap enough. Runs fine, but there is a slight whine thats probably coming from the front drivetrain. Should I take this as a early warning for the 4matic needing rebuilding? Is it the front differential that usually goes bad on the early 4matics? I surprisingly couldn't find much depth on this site for what actually goes bad