Getting an odd knocking noise when gassing/letting off gas. Otherwise engine generally sound smooth (aside from being diesel like).

recently took a video of the engine shutting down and hear some knocking

is my sc coupler toast?

 

I suggest you double check with a stethoscope and also listen to the timing cover near the cam shafts.

 

Also, slack the blower belt and see how much rotational give the snout pulley has before engaging the rotors both directions. It SHOULD be very near zero.

 

Let the engine cool right down (eg leave it overnight), take the engine cover off, then start the engine and listen in/around the engine bay particularly on top where the SC is.
Can you hear a distinct loud metallic irregular rattle often described as "marbles in a tin can"?
If yes then the SC coupler is worn.
If no (for example you can hear rattles and ticks but they are regular and rhythmic) then it's not the SC coupler but something else, possibly timing chains and/or timing chain tensioner(s) and/or high pressure fuel pump(s), all of which can rattle when worn or faulty.
But those other rattles are all regular/rhythmic while the worn coupler rattle is very irregular, I call it "all over the shop".
The most common cause of regular "rattles" is probably the fuel injectors but they tick tick tick (albeit loudly) rather than rattle and the racket they make is normal and unavoidable. The high pressure fuel pumps (two of them down low on the RHS of the block) are also a little noisy, they tick a bit even when new but if/when they wear too much they begin to rattle.