I wouldn't even venture a guess without listening to it firsthand. No guarantees without looking at piston skirts and then measuring the pistons and bores. I'm not suggesting this but bearing knock and piston slap can also sound like one another at first. Worn rod bearings soon get a very distinctive sound and are most often noticed at a lower RPM. The second video in this thread was hard for me to hear what I wanted to hear except in one certain spot. It sounded to me like it had some top-end noise (valve-train) but from underneath I could hear a noise that I couldn't distinguish.
Years ago, I was building 400 Ford engines with 351 Cleveland parts. These were real runners they'd put down 500 usable street HP. These engines would really run with large port, big valve, closed chambered heads on them. We tried parts from several manufacturers including cams pistons etc. We had one that was running a TRW Forged Piston/Piston Ring pack.
This engine was built as well as human hands could put it together following parts manufacturers very closely. It developed a noise (later confirmed as piston slap and beyond) very early in its street life, just a couple of thousand miles. I saw serval of these go beyond 70,000 miles (that was respectable back then for a forged piston engine), including my father-in-law's F150 4X4. It had been broken in properly. Its cylinder-to-wall clearance was perfect according to TRW's specs.
This engine was disassembled it had literally broken the skirts in two cylinders but the break (upside-down triangle break) was such that the skirt was still going up and down with the piston. The cylinders were undamaged. After many discussions and mailing some parts back and forth, TRW re-designed this piston including additional skirt ribbing and a "thicker" ring pack. We ended up using custom Mahle's after this happened. I don't know if the problem was resolved.
I'm telling a long story here but the thing is you could barely hear this noise at first and it ended up literally being broken pistons. Without revving it to a couple of thousand RPMs and backing out, most simply couldn't hear it. I couldn't hear it at a sustained RPM. It got louder but it wasn't like it was just slapping one day and broken the next, very subtle over a few weeks' time. These noises don't go away, they'll get louder. I'm sure that's more than any of you wanted to read.