I have the same thing on the 4-3 downshift. It seems when braking that the downshift sometimes occurs a little late and then it bangs hard. It has been doing this for a while, with no warranty on the car now. It functions perfectly fine otherwise, so I not worried on my car. But your car has a warranty. The downshift will be noticeable, but if it is really hard, then I might want to see what the dealer says.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record