I found the grating web site.

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http://www.edmunds.com/ownership/driving/articles/43814/article.html

When ABS engages, a loud grating sound and rapid brake pedal pulsation often accompanies the pumping action. When this occurs, insurance companies and safety experts theorize, drivers assume that something has gone wrong with their brakes, and they begin to pump the pedal or simply lift off the brake pedal, which defeats the system.
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Then why did MB not include comment about grating sound?