The sound of rain is actually a pretty good one. I find it very calming, and reading on a rainy day is one of the most relaxing things I can think of. Either that, or that squishy-squeaky sound you get when you rub packaging peanuts together or rip them apart.
It's an interesting choice I think, and I understand why you picked that. I was more expecting answers like single sounds however, and since music is usually made up of numerous sounds (not to mention that all music sounds different) I want you to pick something else as well. This time a single sound, something like children's laughter, the sound of crickets on a warm night or even the snoring of your wife/girlfriend, something like that. Think about it for a while and then I want to hear nothing less than what you think is the most beautiful sound in the world. I'm very interested to see what you can come up with