When I change between P, D and R gears.. car jerks just a little.
 

Should I be concerned? I heard it could be something wrong with gear box?

Sent from my SM-G930F using Tapatalk

In addition, after parking my car, when I turn off my engine, my car rocks back and forth.. anything to worry about?

Sent from my SM-G930F using Tapatalk

A small jerk is normal when changing an automatic transmission's shifter between forward-neutral-reverse while braking the car still. You should also never be applying any throttle when shifting from neutral into forward or reverse (and engine should be at idle speed).

Basically, the drive axles are applying a bit of torque, which will cause the engine and transaxle to rock in an opposite direction against the engine/transaxle mounts a small bit. An abnormally delayed jerk, (longer than about 1 second) or too strong of a jerk, could be a transmission problem.

When switching the automatic transmission into "park", there is a lock in the transmission that engages. The car could still however, rock back and forth a small bit due to the rotational slop in driveline components, such as the CV joints. When you use your parking brake, it brakes the rear wheels directly, eliminating this driveline slop. The parking brake is also highly recommended as a redundant safety feature whenever parking the vehicle on an incline.

Hi, the car used to rock a little and I didn't think any of it. Yesterday, it rocked a lot more (but just once, usually forward) for one second each time I switch between Park and Reverse..

My brake is always applied, no acceleration.

I'm a little concerned because my car was the one with a coolant box leak resulting in mixing of atf fluid and water previously. Atf has been replaced and the mechanic says gear box had no issues, could the problems be only surfacing now?

The episode had been about 2-3 weeks ago.

Sent from my SM-G930F using Tapatalk

Take a deep breath and relax. Is the Boxster your first car?

Yes my first car :(

I'm worried about this jerking problem.

Sent from my SM-G930F using Tapatalk

It seems that if I brake very hard.. the jerk is less pronounced. Hmmmmmmm

Sent from my SM-G930F using Tapatalk

Did this excessive, and momentary (you mentioned <1 second long) jerking, which if I understand you properly only has happened once, occur while shifting out of park?

It's possible after shifting into park, if the wheels were under torque (e.g., pressed into a curb, or holding the car against a hill), and then especially if you set the emergency brake like that, and left the emergency brake set after starting the car back up the next time and taking it out of "park", that it could cause a pretty significant and sudden jerk/bang sound to get it out of park. It could also be physically difficult to get the car out of park, in this sort of situation.

Best parking practice is to avoid stressing the parking pawl by keeping the brakes (footbrake and/or emergency brakes) applied during and after putting the gear into "park", and keeping the brake applied until moving it out of "park".
https://en.m.wikipedia.org/wiki/Parking_pawl

But this sudden release of the parking pawl when under toque, if you just did it once or a few times, probably won't have damaged anything, so you don't need to worry about it if that's what happened. (But, do change your parking technique so as to not make it a habit. ;) ).

I hope I understood your description correctly, and that's all your problem was, but if you're concerned, you might want to ask someone to check out your car who is knowledgeable. I hope you can find a better mechanic than the guy you wrote about in this thread!

Best of luck to you!