I've spent the last ten minutes trying to come up with the right word.
Pop?
Blip?
Whoosh?
Which is the best? I can't even tell, any more. ;_;
On the plus side, my search did lead me to
this wikipedia entry on "Cross-linguistic Onomatopoeias", which is quite entertaining.
Apparently, NOM NOM NOM is pretty much universally accepted as the sound of food being eaten. Hee.