Hey I keep hearing this word that sounds like "achehhh". Is it some kind of expression of disbelief?

The word is "adye"; it's an interjection used to express sympathy, shock, surprise
It can translate Alas! What a pity! Poor thing! Oh dear! Gee!, etc...
You might hear it as

Adye o!
Adye mezanmi! 
Adye Bondye! 
Adye widan!
Adye frè!
Adye sè!
Adye manman!
Adye papa!
Adye e e!

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words