Is there some beeps off the throttle body after it?
Does the car stop as you turn it off or judder?
Do you have the aircon turned on?
You have checked the oil level
I've only ever heard the knocking from inside the car as it never does it when i'm trying to hear it, i have heard the beeps but no knocking if that makes sense??
The engine turns off as normal with no judder and the knocking comes after, like something is still spinning for a few seconds.
Air con is not on when the car is turned off.
Just been to check and the oil level is normal.
It's only done it twice, once after a 200 mile drive to Devon in May and the other night after driving 6 miles from the cinema (Man of Steel is rubbish btw!) otherwise it's completely normal?

??