The sound one makes when something is awesome enough to bring one joy to the point of intense sexual arousal.
Originates from the "so i herd u liek mudkips" meme, where in the original story a child is presented with a Mudkip Pokemon plush toy begins to hump it wildly while making an "EEEEEEENNNFFF!" noise.
Natalie Portman and Mila Kunis do it in The Black Swan? EENF!