Engine knock on startup

I suspect you are correct in that the drain back valve is not doing its job and it is starting up dry. I try to only run OEM filters on vehicles that have a history of being "problem children" like the CV. You could connect up a mechanical oil pressure gauge to confirm it or swap the filter for a Motorcraft unit and see if it goes away.

One option that is not a difficult add on would be an oil accumulator. It collects pressurized oil and releases it back into the system if there is any cavitation or pressure loss in the oil system. I have put a few on ex-police vehicles that became fire service cars. You use a solenoid valve to connect them. In use you start the engine up, push the prime button and let it charge the accumulator. Release the button, now check your oil level and top it back up. From then on when you do a cold start you push the button and hit the key, the accumulator will dump the oil into the engine to pre-lube it, then as soon as the engine is running okay you push the button again and step on the throttle a bit to recharge the unit. I added them because the cars could easily set a week or two and it did not seem to matter which filter was on it, they would rattle on start up. They did not with the accumulators but you had to remember to push the button both times, the second time is more important as you have extra oil in the system and it could cause an issue if it gets splashed around by the crankshaft.

Yes, knocking means metal on metal contact. Even a few seconds at 600 rpm is bad. It can easily lead to a spun bearing if the bearing metal galls on the crankshaft.
Good to hear the filter did the trick.
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