I think I should use the because I am talking about a particular instance of rain.
You're talking about a particular instance of rainfall, sometimes called a shower or a storm, depending on quantity, duration, and intensity. If I had
just become aware of the fact that it was raining, or of the fact that it was raining hard (I had known that it was raining, but not that it was raining hard), and chose not to speak naturally by saying "It's raining (hard)," but instead to begin the sentence with "rain" as the head of the subject noun phrase, I would naturally use "the" in the case where I already knew it was raining and wanted to express that "The rain is falling hard" or in a case where I felt a certain kinship with rain and perceived it as
a being in its own right: "The rain is falling."
If I did not feel that kinship with rain, I would say "Rain is falling." It wouldn't make sense to say "
Rain is falling heavily/hard"
out of the blue. It needs to be established, at least in the speaker's awareness, that it is raining before he or she will be inclined to use an adverb like "hard" or "heavily," though a nonrestrictive construction is certainly possible: "Rain is falling -- heavily." Or, in the case of kinship with Mother Nature: "The rain is falling -- heavily." I wonder if more people feel that kinship, at least linguistically, with wind. I think it would be weird to hear someone say "
Wind is blowing" in any context. It's grammatical without "the," but it sounds as though the speaker is on drugs.
Well, I didn't mean to wax so philosophical. But this time you didn't duplicate your question where I usually respond to you, where I am not Phaedrus.