Haha, "no niin" seems to function a lot like several similar phrases in different Canadian provinces:
Nova Scotia (especially the more northern part of the mainland and Cape Breton Island): inhaled "Yuh". Impossible to convey in writing, it's part audible inhale, part whisper. Can be used to indicate assent, surprise, or lack of surprise, among many others.
New Brunswick, at least in Carelton County: "Yes sir". Emphasis on yes; sir usually almost inaudible. Most often used as filler when the conversation lags.
Newfoundland & Labrador: "Yes b'y". Every bit as versatile as "no niin". May indicate agreement or assent, but more likely to show surprise, disbelief, disapproval, pleasure, etc.... or act as filler when the conversation lags, although that doesn't often happen.