I know that it refers to eating and as far as I'm aware, it is quite recent but does anyone know the origin of it?
The full form is Om nom nom nom. Knowyourmeme.com says:
The phrase is believed to have its origin in the sounds made by the Sesame Street character Cookie Monster when eating cookies. In the contemporary lolcat usage, the “om” is sometimes omitted, especially when in the middle of the act of chewing as opposed to taking the first bite and then chewing.
“Om Nom” was the first derivation to appear on Urban Dictionary, with the earliest entry dated December 19th, 2004. This entry defines the word as:
An onomatopoeical adjective based on the sound emitted when something is “oh so tasty” (either through hunger or flavorological value) that one gnaws through it without regard to cleanliness or etiquette. This sort of ravenous eating will often result in an “om nom nom nom” noise being emitted from the eater.
*Om nom nom nom..." originates from a famous trait of the Cookie Monster (a muppet) on Sesame Street.