Noncount nouns almost always have no plural form, so you should treat them as singular nouns and pair them with singular verbs.
"The rain are washing my filthy car for me."
"The rain is washing my filthy car for me."
The first sentence is incorrect because it pairs rain, which is a singular noncount noun, with are, which is a plural verb. The second sentence is correct because both the noun (rain) and the verb (is) are singular, no matter how heavy the downpour.