bad engine rattle series 1 S/C

Well out of no where yesterday the engine started to rattle a little than turned into a lot. All at idle or slight off idle.Not really a knock sound. I have worked on cars for years so i kinda can tell the difference. Deffinitly in the motor took off the belts and noise is still there though just not as much. Checked the flexplate but not broken. Any known issues or maybe balance shaft? This is not really a knock just a bad rattle. And when you drive above like 1500 rpm noise is gone? Thanks

This sounds like the harmonic balancer may be the culprit. See if you can rotate the outer ring on the balancer with your hands. If the rubber insulator has separated between the inner and outer rings, it will make a hammering sound @ idle and up to 1500+ rpm.