There is actually an alternative version of this movie, which had different noises for the aliens, prior to release. The noises did not make into the final product. The aliens sounded more demonic than the mostly (although equally scary) alien noises they ended up having. It alone could have gotten the movie an R on the basis of "terror" alone. It was mostly edited out to keep the movie from being too dark, although some of the things about the aliens were demonic, and kept in the movie. "Signs" got away with a lot for a PG-13 movie. To find them, via the comments, go to the Youtube video to the "Signs" trailer, courtesy of user Rob Jackson (the Rotten Videos group's copy of the video; just type "Signs 2002 trailer Rotten Videos," and look for Rob Jackson's comments, they have the links to some of the clips from that version; courtesy of user Kaylin Starlight).