I have a friend who is having trouble with his mustang. There's only about 100k miles on it despite being a 12yo car. It knocks (once) when you slow to a stop, not during acceleration. Doesnt matter how gradual or sudden the stop is, it happens all the same. I had him try to shift manually. It doesn't appear to knock when he slows down gradually from 2nd to 1st and then stop, only when he is in "drive."

It also idles 500 RPMs higher when shifted into neutral or park and don't ever climb down. When you shift from there to drive the car jerks forward due to the change in RPMs. The same jerk appears when he shifts back into reverse from drive, and is a lot more noticeable when the engine is hot.

Any help will be much appreciative. Thanks.

 

Check all the mounts.

 

Possibly a broken mount. That will cause a jerking motion.