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1. Select a layer in the Timeline panel, and choose Effect – Noise & Grain – Fractal Noise.
2. Set two keyframes for Evolution.
3. Adjust the time between keyframes and the number of Evolution revolutions until you are satisfied with the animation of the noise.
4. Select Cycle Evolution.
5. Set a value for Cycle.
The evolution completes the number of revolutions you specify for Cycle in the amount of time determined by the distance between Evolution keyframes. Determine the Cycle value by considering how much of this cycle you need to render before it repeats. Choose the shortest length appropriate for your project to save rendering time. Initially, the last frame of a cycle is identical to the first frame. To create a seamless loop, skip the last frame by setting the Out point of the layer one frame before the last frame of the cycle:
6. Move the current-time indicator to the time where the cycle completes. For example, if the Cycle is set to 2, locate the frame when the Evolution value is 2.
Note: If you set keyframes for other Fractal Noise controls, return them to their initial settings where the cycle begins to repeat in the timeline, or the controls don’t loop.
7. Move the current-time indicator back one frame.
8. Trim the Out point of the layer to this frame.
9. Pre-render this layer, and import the pre-rendered movie into your project.
10. Select the imported footage item in the Project panel, and choose File – Interpret Footage. Then set Loop to the number of loops required for the duration of the layer in the project.

via Adobe After Effects CS4 Fractal Noise effect.