Yep when I'm starving I sometimes make that food, felids and canids can also do similar, the opening om sound is the square like sound (not quite sinuisoidal as the om sound in chants which is done with a stronger vibrato) of you opening your gullet while there's still air in there and gulping the food down at the same time, the n sound is the air trapping in your gullet and from your mouth closing in sequence to you re-opening the mouth/gullet. It's a rarer sound to make due to etiquette but the Cookie Monster from Sesame Street was renown for it also as well as some cartoon characters which made it popular as an expression, I guess. Um wait, why did I research the origins of nomming? I wonder if one can get a degree in nomology.
*noms the serval and nixon*