This sounds like the Anti-Lock brakes are "kicking in". When they decide to take over, it feels like something is grinding your car to it's death.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles