Normally audio comments like this from you and AFE annoy me slightly (coming from a no budget background so as you know sound is often cheaped out on) but this is an example where I thoroughly agree that the sound could very much make or break your fake rain.
EDIT: Oh and I know that sound is important, it's more that I get frustrated that I can't afford to lift the quality of my overall sound mix
Check out the DVD extras on "Forrest Gump" where Randy Thom discusses the Viet Nam rain sequence, and the challenge of making the sound of different types of rain (Forrest - "
One day it started raining, and it didn't quit for four months. We been through every kind of rain there is. Little bitty stingin' rain... and big ol' fat rain. Rain that flew in sideways. And sometimes rain even seemed to come straight up from underneath. Shoot, it even rained at night... ")
The "secret" is literally in the details. Sonically rain comes across as a lot of white noise, so you need to add "close up" details to provide more information. In the film "TrailerPark" I used the detail solution as a part of a thunderstorm; the scene was indoors, so I used "window splats" (shooting the hose at my own windows) and a gutter downspout (again from my own house) to enhance the ferocious rainstorm idea. The thunderstorm was also from my personal collection.
This applies to many more situations, and preplanning the audio post of your film during preproduction can allow you to do things that you might have thought beyond your budget. In the film "11" they shot a "huge" political rally with only 50 extras by keeping it POV from close to the "stage." After the shot was completed the sound team got the extras to do multiple audio takes of cheering, chanting the "candidates" name, etc. I put all of that into layers and supplemented it with more crowd noise (library) and a couple of loopers. The result was quite gratifying.
You can do the same with a restaurant, transit station (train, bus, airport), a crowded bar and lots of other locations. By shooting close and judicious use of extras, who have multiple changes of clothes, sound can substantially support the illusion. They used the bar solution in "TrailerPark" and when the shooting was completed opened the bar. People trickled back in after changing back into street clothes. They kept the juke box shut down for an hour and I got a lots useful stuff, from a couple of people to a several dozen (including crew and significant others), so I could have a quiet Tuesday night or, by layering, a packed Saturday night.