The sound for equipping armor played whenever i got hit by a projectile. I don't know if this is intentional for detail in sounds, but maybe it is. Also the sound plays when other things happen.
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ampolive
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We're tracking this issue in MC-251859, so this ticket is being resolved and linked as a duplicate.
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I think it's triggering when the armor's durability changes, the same way that an item in the off-hand animates when the Mending effect is triggered.