I am a retired Motor Engineer with experience on Twin Cams and Cortina engines in competition.
If you insist on doing your own diagnosis with the aide of others I suggest the following. Has the Oil pressure altered from the norm at all ? If so look to mains or a rod cap loose.
You indicate the noise is there while cranking the engine on the starter. Remove the spark plugs and try, if the noise is different in tone or volume then it is internal. Also try with the clutch depressed looking for a softening of the noise.
Next remove the engine mounts and suspend the engine on a jack ensuring it it is secure, CRANK the engine, plugs out, and note the noise. Next lower the engine say 1", try again and note the result. Lift the engine 1" above the original line and note the result. That should eliminate contact type noises / problems.
Once you have eliminated any chance of a contact type noise then remove the engine and have it stripped and inspected by a GOOD mechanic. From now on I'm looking at a loose flywheel or possibley a broken crank although that is uncommon with the Ford based bottom end.
It is very difficult to diagnose a problem on the desciption of another person, my idea of a knock is different to yours as is the volume.
PS. Another possibility is the Crank thrust bearing breaking up and slipping out. Depress the clutch.Then, go to the front of the engine and see if you can move the crank back towards the rear. If so, probably 1/8" or so, then then the thrust bearing is the problem.
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