The sound "minecraft:entity.player.attack.weak" and "minecraft:entity.player.attack.knockback" is played when the player attacks any entity that is currently invulnerable. Currently this sound is supposed to be played when the player attacks any entity with an attack that isn't fully charged to indicate that the attack dealt less damage than it can or when a attack is made that dealt substantial knockback to another entity. 

This leads to where it can occur quite often in situations such as: 
-> When the player attacks any entity during a invincibility tick. This is most notable when attack speed is set to a high number to remove the cooldown and the player continuously attacks another entity, creating a deafening noise. 
-> When the player attacks any entity that is set as invulnerable. 
-> The player is on the same team as another person and they cannot receive damage.
-> Servers where large groups of players congregate and do not have pvp enabled.