The pitch contour of the interjection ううん has been described as high, low, middle. Alternatively, it has been described as a falling-rising tone. In either case, this appears to be unique in Japanese.
In English, the pitch contour of "I donno" is distinctive. I can reduce it to "ionno", "ionn", "nnn", or just hum the pitch contour without opening my mouth, and it's nevertheless understandable.
Given that the pitch contour of ううん is unique, and given that it's already fairly nasal, can I do the same thing? Can I say んんん without opening my mouth at all? If so, is this the normal pronunciation?