3.8 Mommyvan off idle knocking...

Remove the plug wires one at a time and see if you can pinpoint what cylinder its on.
If the knock doesn't go away with one wire, it could be something totally different.
Those alternators have some sort of clutch drive behind their pulley. Those will scream sooner than knock but could create a knock sound in belt tensioner reaction.
Who knows.
From my old school experience...Timing chains did slap the inside of the timing cover. GM timing chains would wear through their sheet metal cover and toss all the oil before the chain would jump a tooth.