The Add Noise filter is accessible in the Noise category in the Filters menu. This filter concentrates on single pixels, increasing or decreasing their color values and thereby creating "noise" in the image. Noise can be defined as those single or small groups of pixels whose color values differ considerably from those of the bulk of nearby pixels. The overall effect can be like the grainy look of old film.
In the "Add Noise" dialog set the following options:
In the Parameters box:
Use the slider to set the Amount of variation from the original colors. The lower the amount the closer are the pixels to the original image colors.
Tick the Monochromatic box if you want to create noise that lightens or darkens pixels but leaves their hue unaffected. This results in most pixels being either black or white when Amount is set to high.
In the Distribution box choose the type of random distribution to be used for the noise:
Uniform leads to a random distribution of even density throughout the image.
Gaussian leads to a much more random distribution than Uniform, in which noise can be concentrated in single pixels and groups of pixels, and there can be areas of no noise at all.
All the changes you make can be seen immediately in the Preview box.
See: