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

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80 280TE, 5 speed
83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts