Makes chattering sound. Surprised me on my first use of system.
My car has 65000 miles.
My car has an automatic transmission.
Hello - the Downhill Assist Control (DAC) uses engine compression (without ignition) to provide "braking" without heating up the brakes on a long descent. This makes a somewhat "unnatural" sound much like you describe from the exhaust. It is not harmful, but is perhaps a bit unsettling since it is not what you would be used. If you feel this noise is exceptionally loud, I recommend a noise inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, check for any problems with the DAC system, and give you an accurate assessment of damage and cost estimate for repairs.