The Bug:

Horses play armor equipping sounds when their armor item is read from NBT.

If a horse has armor equipped, it'll make noise when you change its NBT data. 

Steps to Reproduce:

1. Summon a tamed horse that has some armor equipped on it by using the command provided below.

   /summon minecraft:horse ~2 ~ ~2 {NoAI:1b,Tame:1b,ArmorItem:{id:"minecraft:diamond_horse_armor",Count:1b}}

2. Modify the horse's NBT data by using the command provided below and listen closely as you do this.

   /data merge entity @e[type=minecraft:horse,limit=1,sort=nearest] {FallFlying:1b}

3. Take note as to whether or not horses play armor equipping sounds when their armor item is read from NBT.

Observed Behavior:

Horses play armor equipping sounds.

Expected Behavior:

Horses would not play armor equipping sounds.