Dic: grapevine, hear on/through the grapevine

• I heard on the grapevine that she was pregnant, but I don't know anything more.
• I had heard through the grapevine that he was quite critical of what we were doing.

2(a). The informal transmission of information, gossip, or rumor from person to person.

2(b). A usually unrevealed source of confidential information.

.... The American Heritage

2. Informal | an unofficial means of relaying information, esp from person to person

.... Collins English

grapevine - gossip spread by spoken communication; "the news of their affair was spread by word of mouth"

pipeline, word of mouth

scuttlebutt, gossip, comment - a report (often malicious) about the behavior of other people; "the divorce caused much gossip"

..... http://www.thefreedictionary.com/grapevine