1. Consider the possibility of being overwhelmed by moe to the extent of having certain or multiple internal organs imploding.
Hnnnnnnnnnnggggg!!! can be considered as a reaction to such incidences of moe overload.
2. Can also be consider as the sound you make when you feel indifferent or annoyed by something
OMG! Azu-Nyan moe' gallery!! I think I... hnnggg!!!!!!!
a. I finally went out with that chick i told you
b. Oh really... hnnggg...