its easy. well no, you have a new layer right?..you add noise 23% or so then you do a gaussian blur at and angle like / <---that.
now it might be fussy, I am not sure if you have to like rasterize the layer, but I know it didnt work on a transparent, so I just did a black back ground, with white noise, and then did the rest. When it was dun I just change the layer blending, forgot which one, but its easy ans quick to do so it doesnt take long to figure out.
better Idea Il lget you the link of the tut. in an edit of this post.
http://www.subliminalgenesis.com/tuts/ps tuts/rain.html
good place, nice forum. par, and fyre and pretty nice guys.