Sounds are kinda hard over video. Obvious thing to check would be exhaust, but the fact it goes away after you rev it up seems like a collapsed lifter to me. Once you rev it and build the oil pressure the lifter pumps back up and quiets down.
Use a long screwdriver as a stethoscope and move it around on the intake and valve cover to see if you can track down the source of the noise any better. Then pop a valve cover on the noisy side and rotate the engine to verify. You'll see one rocker not sawing as much as the others.
You can buy the roller rockers individually, but even with the roller tip you'd better track down the source 'cause you can still wipe out a cam lobe it it really is a bad lifter.