Need help finding a word?

Hi, I am trying to find a word for the noise a person makes when they're in disbelief.

It is sort of like exhaling hard, or a cross between a "tch" sound and huffing--if I had to spell it out, it would be something like a "thuh" (NOT a "th" sound, all the letters are seperately being pronounced).

I don't know if there's a word for that, but if anyone knows it, or knows a word that means something similar to what I'm describing, please let me know!
thanks alot for this, great site!