grapevine

1. N-SING If you hear or learn something on or through the grapevine, you hear it or learn it in casual conversation with other people. (消息、传闻等的) 私下传播途径

I had heard through the grapevine that he was quite critical of what we were doing. 我已经私下听说他对我们的工作十分挑剔。