You're probably looking for a soundscape, so here's a tutorial on them
Theres also an entity called ambient_generic for minor sounds (door closing, etc.) but since you're talking about rain sounds, a soundscape is the way to go. Soundscapes are like the environment sounds of your map or constant background noise.
The Valve Developer Community is the Wikipedia of hammer and source modding. Everything you need is there, although it's very technical sometimes.
So follow the video, and you can look here for
a list of TF2 soundscapes to use.
The VDC also has a library of everything else you need to make a TF2 map:
click here to see the page
Make sure you bookmark this page! You're welcome to ask questions here at TF2m as well, although we will probably just tell you to visit the VDC.