One possible source I haven't heard mentioned is the front derailleur cage striking the back of the crank. If it were very close to hitting under no load, flex in the frame and various parts could bring them in contact under hard pedaling - only when on the big ring of course.
At some point ('78?) the space between the crank and the big ring was increased. If you have the older crank, you should use the older front derailleur with no bent-out lip on the cage.
Mark Bulgier
Seattle, Wa USA