I know that ふ is normally pronounced as [ɸɯ]. But ふ is sometimes used to represent a clearly different sound that occasionally appears in spoken Japanese, and I'm having trouble identifying its IPA representation.
The following is what I know about this sound:
It does sound like the h consonant to me, but it can be produced with your mouth completely closed.
No normal Japanese word contains this sound, but a few interjections and onomatopoeias like ふーん, ふふんっ, んふふ, ふっふっふ, ふがふが are often pronounced with this "sound".
It's the very first sound in this video. It's written as ふーん but it's obviously not [ɸɯ].
Japanese Wikipedia suggests this sound is 無声鼻腔摩擦音 ("voiceless nasal fricative"?).
But the article gives no IPA representation, and this term appears nowhere else in Wikipedia.
I think English "hmm" and "humph" have a similar sound, and some sources including this suggest this sound is [m̥] (the voiceless version of [m]). But other sources including this and this seem to suggest [m̥] is a rare consonant.
So what is this sound in IPA? How do you transcribe ふーん and んふふ said with your mouth closed?