I hate the doomsayers on Facebook who always assume the worst, and declare every lightly used part to be complete garbage, but….
That sounds far too harsh to be piston slap, and the frequency is wrong.
Piston slap from wear will occur pretty much evenly across all cylinders, and occurs twice per revolution in each cylinder (TDC and BDC). As such, it tends to be a “thrashy” noise, kind of like someone with Parkinsons perpetually tipping out a bucket of gravel.
Your noise sounds much harsher and lower frequency, like a problem in only one cylinder and a problem larger than only one cylinder with excessive wear.
The clean oil and the fact that it diminishes when warm, suggests that it isn’t a bad big end, but it is still an outside possibility. I would be expecting a problem with just one cylinder.
You also need to give it a proper rev, not these quick blips. A bag big end will reveal itself then, but can hide during a quick blip: I had a Mitsubishi engine that would sometimes be quiet (not perfect but acceptable) on cold start up, and would stay that way until it was held over ~3,500rpm for a couple of seconds, or it was revved over 5,500rpm.
Unless the audio is deceiving, I really don’t think this is just piston slap.
