So i recently got my 1996 Miata running on 7 psi of boost and i ran into this problem, when the engine is running everything is great, but when i turn it off, about 2/3 of the time it sounds like its knocking 4 or 5 times as the engine stops running. Yet there is no knock whenever its running. Does anyone have a similar issue? Its running 10w40 oil if that makes a difference.

 

Consider doing a compression check to see if there is a notable difference between cylinders. You may find a stretched or bent rod or pistons rocking in worn bores as a noise source.

Sometimes a spun bearing can be noisy on decel or ignition off without knocking under load.

 

My VVT swapped NA does this same thing too once the oil is warm and I shut the car off. I was concerned at first but after 5k+ miles the car still runs fine and still makes good power. /shrug Only a very slight amount of metallic sheen is found on each oil change.