"Co to za" is a whole expression where you are askking about the source of sensation (sound, smell). Without "za" this question would translate as "What is a sound?" – as if you've never known a word "sound".
Nice, I like the literal translation: "What is behind that smell, noise, etc.?"
Funny thing is that the literal translation of "to" is never "is". It's not even a verb.
EDIT: I'm also not sure about translating "za" as "behind", since the case doesn't match (and I don't perceive these two meaning close, even if the word is the same).
Some English speakers say "what is behind that...?" to mean what is the cause...I wonder if this turn of phrase is typically Irish?
I don't think so. I can think of two cases when you use phrase "Co to za dźwięk?": 1. when you suddenly hear something strange or perhaps you are frightened by a sound or noise. 2. when you play guessing game. I am not native English speaker, but I don't think you would use expression "What kind of sound is this?" in any of above cases.
Could you not say 'Co jest ten/tamten dźwięk?', or does that not make sense?