What tappets did you use for the rebuild?
This sounds identical to the problem I had. Replaced cam ,tappets, timing chain etc. Britpart kit.
At first it felt like a different engine, but within 500 miles I had a really bad tapping back on one tappet.
Took the rocker covers off and checked the push rods for excess movement. Found one was really loose.
Stripped the top end back down and found this
the cup in the tappet had spun round causing damage to the cam, push rod, and rocker cup with it all rattling around.
Second rebuild in 500 miles, this time using OEM tappets and another new cam
Having said all that when I watched your video it does look very dry in there. Check the rocker oil ways aren't blocked if you are sure they are mounted correctly