Out of the blue, mine started this at idle, then quit after restart. Hasn't done it since.

 

Weird. I'm no mechanic but is it possible it's the ECU retarding the ignition to such a degree it makes it sound like this?

 

Its the chuffing noise in that recording which worries me ...sounds like air ..loose plug or similar .I would get a length of pipe about 2 in dia and 2 ft long to your ear and see if you can isolate it .
Piston slap is sharper like a bad tappet

 

Get yourself a stethoscope from an auto parts store and start probing. Stop the guess work.