Pinyin vs. Pronunciation of 嗯?
Very interesting question!

嗯 is pronounced as ǹg，ńg，ňg，ǹ，ń，ň. 

ńg, ń: means what？, or 什么？It expresses surprise or raise a question.

ǹg，ǹ: means I get it, I know it, or 好，我知道了

ňg，ň: means no, I don't want that, we say 嗯，不是，... 

The way to pronounce ǹ，ń，ň is that you put your tongue in the place while you pronounce the phonetic notation /n/ in English and then apply the tones. 

To pronounce ǹg，ńg，ňg , put your tongue in the place while you pronounce [ŋ] and then apply the tones. 

Of course, ǹg，ńg，ňg，ǹ，ń，ň are unusual pronunciations in Chinese.