First motor start, terrible knocking

[Lefkada]

Fired up the motor today for the first time to hear a horrendous, fast-paced knocking. We quickly shut her down and pulled off valve covers, but didn't see anything obvious.

Engine was partially rebuilt. I pulled the heads off for a valve job but didn't touch the cam or pistons. This sound did not exist prior to tear down.

I'll be pulling the pan tomorrow but was wondering if anyone knew immediately what may be producing this knocking?

https://youtu.be/VAxGZIOPXL0