It's sounding like the dreaded rod knock that's all to common in these engines...
Normally the chain tensioners and/or lash adjusters would make their noise at idle as well (but not always). The bottom end rod knock is easy to tell as it will do it under no load too (free revving in neutral) The chain tensioners and lash adjusters will less the noise under no load.
Checking the plug gap won't help here since the piston would smack the head before touching the plug, though severe detonation would run it white if that's the direction you were hinting at...
