OK, I hadn't made an animated GIF before. I found this tutorial (http://www.planetphotoshop.com/animated-rain.html) which is basically similar to the effect I wanted to achieve however there were 2 problems that I ran into
1) The rain gif isn't transparent, it shows a black border
2) The file size of the completed GIF was massive, nearly 6MB
Not sure where I am going wrong as not done this before?