it never rains,but it pours

本来从不下雨的,却下起了倾盆大雨
是祸不单行的意思

不鸣则以，一鸣惊人

it never rains, but it pours
这儿要么不下雨，一下就是倾盆大雨。

要么就不下雨，要么就下倾盆大雨！

字面意是“不下则已，下则倾盆”，翻译成相应的成语就是“不鸣则已，一鸣惊人”。
IT NEVER RAINS BUT IT POURS -
"One stroke of good (or ill) fortune is often followed by many other instances of luck (or misfortune) when you least expect them. The proverb dates back to the eighteenth century. In 1726, English physician John Arbuthnot (1667-1735), published a book entitled 'It Cannot Rain But It Pours.' Jonathan Swift (1667-1745) and Alexander Pope (1688-1744) collaborated on an essay entitled 'It Cannot Rain But It Pours.' The saying has been use ever since." From "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996). The saying, in a slightly different form, is the slogan for Morton Salt: ".The company developed a salt that would be free-running even in damp weather. In 1911, a little girl with an umbrella and her now-famous slogan, 'When It Rains It Pours,' were created to promote this new product in a national consumer advertising campaign. The Morton Umbrella Girl and slogan first appeared on the blue package of table salt in 1914. Throughout the years the ageless girl has changed dresses and hairstyles to stay fashionable. She was updated in 1921, 1933, 1941, 1956 and 1968. Together they have symbolized the growth and progress of the company through the years."

不雨则以，一雨倾盆。

这儿从不下雨，但是会下倾盆大雨。

不鸣则以,一鸣惊人
这是一句谚语,与之对应的意思就是这个了.

从未有零星小雨，而总是倾盆大雨