I was performing some Catford-style "experiments" with nasal consonants, and found that slight opening of the mouth or rounding/unrounding of the lips has no particular sonic effect on, for example, [n], [ɲ] or [ŋ]. That matches intuitively with my understanding of nasal consonants, since it doesn't seem possible for any changes in front of the point of articulation to have an effect on a sound with no air past the articulation point. However, whenever I try to produce [ɴ], rounding/unrounding and mouth movement have a relatively strong effect—one that sounds a lot like a shifting formant. I hear no fricative sounds, so I'm virtually certain that I'm not accidentally producing some sort of nasalized [ʁ]. I suppose that leaves the possibility that, depending on rounding, I'm accidentally producing a nasalized back vowel such as [o] or [ɤ] (apologies, I haven't figured out how to add the nasalization diacritic).
I looked at the section in Catford on uvular consonants, and although it does mention a certain "messiness" for the stops [q] and [ɢ] (which seems potentially significant to my problem), it describes the uvular nasal as unproblematic and easily found by producing [ŋ] and simply lowering and bringing back the tongue.
My main question is: is this just the nature of [ɴ], or should it be possible to produce it in a manner such that changes in the lips have no effect? Secondarily, if indeed an ideal [ɴ] is possible, do you have any advice on how to produce it properly?