Somewhere around forty minutes into Kizumonogatari: Tekketsu-hen, Araragi is set upon by the three exorcists who earlier dismembered Kiss-Shot: Dramaturgie, Episode, and Guillotine Cutter.
As they advance upon him, they speak to one another - but all we (the viewers) hear is a sort of guttural grumbling. Araragi presumably hears the same thing (non-speech sounds), given that he says something about trying to get them - as fellow human beings - to communicate with him rather than kill him.
What's the deal? On the one hand, this was certainly an incredibly stressful situation for Araragi, so he could well just have been on an adrenaline rush that was interfering with his hearing. But then again, this being Monogatari, it's not implausible that maybe they were speaking some sort of secret exorcist tongue.