on the grapevine
Through or via an informal means of communication or information, especially gossip. There is always some absurd rumor or piece of misinformation on the grapevine at work, but I try to just tune most of that out. I heard on the grapevine that John is planning to propose to Samantha next week!
on/through the ˈgrapevine
by talking in an informal way to other people: I heard on the grapevine that you’re leaving.