The Noise Effect, also known as film grain, is a post-process screen effect that is used in most of the games; it was introduced in Silent Hill 2. It is a screen filter that lets the player know if there are enemies nearby when the screen starts to become static and eerie. It was originally designed to hide the "CGI-like" look of the characters in Silent Hill 2, and to help add to the atmosphere of the games.
In Silent Hill 4: The Room, Henry Townshend being near Ghosts will cause the Noise Effect to become more prevalent. Some enemies are not detected by the radio, like the Stalker in Silent Hill and the Mannequins from Silent Hill 2. The filter is used most heavily in Silent Hill: Origins.
There is usually an option to enable or disable it, although this may have to be unlocked first by completing the game. Many players feel that noise/grain effects should be optional in video games.
Even in Silent Hill 4, if it is disabled in the menu, it still reappears when Ghosts are near.