Originally heard in the game, "Team Fortress 2" as the Heavy class enjoys a sandvich (this is the correct spelling within TF2 due to the Heavy class' accent) all the while saying, "Nom nom nom..." while he consumes it to gain life.
also, "Nom nom nom... Sandvich makes me strong." -Heavy
"Nom nom nom... Balony is perfect fuel for killing tiny cowards!!!" -Heavy
Can be used in an appropriate image file with only the caption, "Nom nom nom"
2). the phrase is near commonplace on the internet but originated with Sesame Street's Cookie Monster
3). the sound is also used in Team Fortress 2 when the Heavy class eats a deliciously looking "sandvich" after becoming damaged and regains full health.
Heavy: "What was dat sandvich? Kill dem all? Good idea! Ahahaha!" (bites sandwich) Nom nom nom...