I had a knock also and it turned out to be my timing gear was worn badly. Here are a couple of videos of the sound the timing gear was making while running, hand cranking with the cover off and after a new timing gear.
The knock:
https://youtu.be/3R3eLKlvVmU
Hand cranking with timing cover off:
https://youtu.be/lDI6grnm-Z0
After new timing gear:
https://youtu.be/C-MMJRXug8E
I hope this helps.
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