You'll want to use a 3d looping sound for the decal, which can be achieved with the "loopsound" command. Simply say "loopsound=<name of sound>", of course, with the proper path to go with it. The path starts from inside the "Files" folder, so if you're going to be using a sound inside the audiobank, let's say, in the "atmos" folder, it'll look like this:
loopsound=audiobank/atmos/sound.wav
As for your rain turning on and off, you have to activate the decal entity that the script belongs to. That'll need to be done in one of two ways.
1) Have another entity's script activate this entity
2) Have this script activate itself
If you want the script to activate itself, then you'll be usin the "activate=X" command, where X is the activatio value you want to be giving the script.
I'm not sure how you want to turn the decal on and off, but if it's with a timer, then you'll need to be aware that you only get a single timer with a non-Modded FPSC, and to get more, you'll need to go through some painstaking scripts using variables. (I believe Conjured Entertainment made one)
And there you have it. Go forth and create!!!
The one and only,