Is the squealing recurrent, or does it happen just once then go away for the rest of the day?

If it is recurrent, you probably have some fluid leaking onto it, like Power Steering fluid or coolant.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black