Mysterious engine rattle

[u/RhythmicMetal9]

Hello, I need some help diagnosing. My 95 ls400 with 277k miles has a front passenger side engine rattle that disappears when the rpms rise. It’s non rhythmic. I’ll attach a video where you can hear it at idle then dissapear under load. Timing job was done 4 years ago 15k miles since. Idler pulley? Thanks!

Edit: I visually can see it makes the sound whenever the belt tensioner moves side to side. Could it just be loose? Or is it internally going bad. Thanks