hmmm...not easy to do.
You can make a semi- transparant grainy texture
and define it as a resourcepic.
But it's very hard to make a good one...
Anyway, Did you ever tried the satellite script from Vectorboson? To simulate TV static he uses 2 simple pics and he plays those randomly in a dialog with a very fast loop to create a bit of animation.
Maybe you can do the same and make those pics for more transparant and use them in a similar way.