It's pretty loud so it sounds like its outside the block. Cracked flex plate? Maybe loosen the fan belt so the stuff on the front of the motor doesnt turn and see if it goes away. If not then it's probably in the bellhousing
Pull the valve cover and inspect rocker arms and bridges to eliminate issues there that could make it drop a valve. What does it do if you increase engine speed? (cut broom handle or anything you have handy. measure between front edge of driver's seat lower cushion and acc pedal then add 1 1/2 inch. Slide stick progressively to the right to adjust speed)
That is not flex plate. I had a 6.0 with that and it was regular beat knock like it is on any gasoline engine. It changed with trans in gear and the loading from that. Loose TC nuts was also a regular beatnoise that didn't sound like that.
After reading your post I figured oh no maybe a failing lifter. But after watching the video I certainly don’t think that is the case as the noise is only ever now and then. I could be wrong but I would do what some of the other posters have suggested first start by taking the fan belt off. Start the truck for a short period and see if the noise goes away. Otherwise also check the area of the flex plate as also stated. If none of this proves to help find the noise I would take a pipe put your ear up to it and move it down the valve cover there. I know that sounds stupid but it may help isolate the noise of the engine and help you detect a possible valve train issue. However I do believe it’s going to be something other than valve train issues. One other possible thing I can think of is a timing gear set issue under the front cover. Otherwise good luck keep us informed on what you find!