Originally Posted by FBinNY

IME the number one cause of chain rub of the FD that can't be detected on a repair stand is crank/BB flex. (this assumes you correctly identified the sound as chain rub on the FD cage).

First check that the BB has no play by flexing a crank and seeing if if the opposite one moves also. If that checks out stand the bike up with the front brake on and the cranks horizontal, and press the right crank with as much of your weight a manageable, while watching for movement of the chaining and chain inside the FD cage. There's always some flex, but you only care if it's enough to cause rub. If so you can usually resolve by adjusting the trim accordingly,