Little story about Forrest Gump and that scene:
Senior year of high school our one English class had us watch it and analyze his storytelling. Now, I had already watched the movie numerous times, it's one I never get sick of. So we are watching the movie, and this scene happens, young Forrest grunts, and the lightbulb goes on in my head. I let out a large "OOOHHHHHH I GET IT NOW!" in the middle of class. The dozens of other times watching the movie, I just assumed this was a joke about Forrest being "dumb" and couldn't speak. But now, at 17 years old, in the middle of a Catholic high school classroom, I finally realized he was imitating the sex noises.
Anyway... Go Irish, Beat Knights!