Thank you for making me actually think about some of the idioms we take for granted. The meaning is the same as saying, The rumour is..." (Rumor is the American spelling, rumour is the British spelling) - and that is what people 'hear' when someone uses those words, rather than the individual words, especially the "it" part. It actually comes from the expression "to have it", meaning "how
I don't know what your native language is, Anon, but in some languages the kind of it you have in your sentence is called a formal object as it is the object of has. Some grammarians call it "unspecified it." Examples of it used as an object without any clear reference abound in English:
Help Me Make It Through The Night (a song) Some Like It