Poor Kisume...

I still can't clearly imagine the noises that they make after all this time.
But because they make a 'sssz' noise, I can't help but think they sound something like creepers X.x;

It's not really a "ssz"; that's a bit too sizzling. The person who uses that translation usually employs it for a "su" or "so" noise, which is more whispery, like a slight gust of air. (Actually, in this case, "so" implies "sotto" meaning "slowly", so it's not really a sound at all.)

I thought the 'sfz' and such noises were noises of movement...

I saw it as just sounds they make like they have their own lenguage