Hobgoblins' eye changes based on whether or not it can use Thermal Barrier. I've killed well over 10,000 Hobgoblins and yet not once noticed this small detail until I was just mucking about with the HUD off.
[spoiler]Left: On cooldown
Right: Active[/spoiler]
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The Touch of Malice is silent when the mag is full, but once you get to the final round and trigger its intrinsic perk, you can hear Oryx's worm whispering to you