Exclamatory: UNN-yuh. Used as an exclamation for something when you are extremely pleased. Can also be used as a negative term if you are let down or if something dis-pleases you. Third usage is if you are surprised.
Usually never used without dragging out either the 'UN' and/or the 'YAH'. It gives it more depth and a fuller meaning and lets people understand how you mean it.
Surprised: (i.e. "BOO"..."UNNYAHH!")
Displeased: (i.e. "we can't go to chipotle today"..."aw man, unnyahh")
Pleased: (i.e. "heres a million dollars"..."UNYAHHHHH"