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2001 I don't think is known for piston slap is it? that is something that seems to stop after warming-up. ugh - beat by 6 minutes!

That sounds like a dead cam belt tensioner. Removed LHS cam cover and check if the tensioner is bouncing. There are plenty of YouTube videos about this. If you can’t see it, it doesn’t mean it’s good, use the above link to the stethoscope and use this to properly diagnose the issue. We had one go bad on our EJ251 and it looked fine while running but the stethoscope pinned the noise with the tensioner. Swapped for OEM and no issues since Cheers Bennie

Cut open the oil filter and check for metal. Look in the pan also. Likely the cogged idler was the problem as you surmise. GD