See also:

Brakes
Occasional brake noise is normal. If a metal-to-metal, continuous grinding or continuous squeal sound is present, the brake linings may be worn-out and should be inspected by an authorized deale ...

Troubleshooting
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Low speed automatic cancellation
ACC is not functional at vehicle speeds below 16 mph (26 km/h). Once the vehicle speed drops below 16 mph (26 km/h), an audible alarm will sound and the automatic braking will be released. ...