In a new comp apply the block dissolve effect to a full screen solid. Set the block size to about 30×30 and turn off soft edges. Animate the transition from 100% to about 70% over 3 seconds. Select the keyframes and wiggle them with a rough setting.
Create a new solid, set it’s in point to 0:0:10 and do the same thing with slightly different block size/timing/wiggle settings.
Do this two more times with solids starting at 0:0:20 and 0:01:00.
Now the screen should be mostly filled with various sized blocks.
Add one more layer and do the same as above except adjust the transition from 100% to 0% over 1 second and slide it so that the last frame ends about the same time as the keyframes of your latest solid.
You can now use this comp as a track matte for anything you want.
Forgive the hasty tutorial but I have to get out of here. If you need more details I can give them tomorrow.
Matt