I have profound noise induced hearing loss in the upper frequencies. But this rattle is lower frequency so I readily hear it. So no, I don't have better hearing and its likely worse.
Yes its the 'moderate' impacts that rattle, not the slight ones. And if it happens with sharp impacts I can't discern it from other coincidental noise. And it happens a lot.
Haven't heard anything further from LBS or Spec. For now I'm just riding the hell outta the bike and trying to ignore the rattle.
I like the suggestion of trying out a $10 mechanics diagnostic stethoscope (which get very mixed reviews) but not sure how I'd ride the bike, hold the tip of the scope securely against the various bike parts, listen attentively, move the tip around, and operate the bike safely while doing all of the above. Perhaps in a rough parking lot with no cars around.