It doesn't sound like piston slapping, because it's not loud and it doesn't sound like knocking. It sounds like a left over lifter tick that I got from running my oil almost dry by accident.
Is there any way to quiet lifter tick safely? I've read about liquids to put into your oil that quiets it temporarily or something. I don't know if that's safe. I know a lot of that sort of "remedy" stuff builds sludge and stuff.