Rain in music
Rain is the most used ambience in lo-fi and chill music, and it works well under intros, outros and spoken word. A steady, mid-intensity rain fills space without stealing attention; a downpour or thunder makes a statement. Field recordings capture the space they were made in, so rain on a tin roof, a car windscreen or leaves each sound distinct.
Keep ambience low and filter the extremes: roll off some low end to avoid rumble and some top end if it competes with hi-hats. Stereo recordings give width that a mono loop can’t.
Sources
Most free rain recordings come from field recordists on Freesound and Wikimedia Commons, many under CC0 or CC BY. Broadcaster archives also have rain, but often under non-commercial terms, which the badge marks in red.