You could try this:
Add a new layer.
On that layer, Effects > Render > Clouds.
Then, Adjustments > Brightness / Contrast.
Turn the contrast up almost all the way and adjust the brightness to taste.
Press F4 (layer properties)
Adjust blending mode (try "multiply" or "color burn"?)
Adjust the Opacity for desired effect.
That should do it!
You can always go back to Render > Clouds and fiddle with settings for different patterns.