This is based on a request from another thread in which Bill politely informed us that it should be moved to a new thread. Thanks Bill 🙂

This is a "thinking aloud" nebulous answer, but hopefully will get you thinking along the right lines.

Now you could record 4 or 5 minutes of Rain and just play it as a sample (like a backing track). But that's going to chew up quite a bit of memory.

Or look at it from a "Sound Design" perspective.

Rain, is basically "white noise". Interspersed, non specific Harmonic spread across a wide spectrum. It is also subject to some natural phasing or Comb Filtering, giving it a sense of moving around, intensifying and lilting as presumably, the wind moves "sheets" of rain around.

You can find "Rain" and "Noise" samples in the MODX Sound FX or Music FX sample library (Waveform Category search).

On their own, these Waveforms sound pretty crappy and "mechanical". But you can Spread, Layer, Randomise and Diffuse them by using Effects.

You can start by adding Chorus with slow speed and moderate Depth to create more "voices". You can then add a slow Phaser or Flanger to create a shallow and gradual fade in and out of "distance".

Add a dense, somewhat randomised, mixture of Delays (try cross Tempo or LCR Delays) and then push it through a cavernous, diffuse Reverb, with a High Wet (maybe 100% Wet) setting. Finish it all off with some Compression and EQ.

Bear in mind, you can have up to 8 samples (Elements) playing, and you can treat each Element to different Filter, Amp and Pitch settings... Envelopes, LFOs etc.

You are aiming to create a "monotonous" background wash of "white noise" that sounds like Rain.

I'm not going to do it for you. But I've shown you were the sandbox is, if you want to get creative.

Have Fun.