A British Airways Boeing 747-400, registration G-BNLY performing flight BA-294 from Chicago O'Hare,IL (USA) to London Heathrow,EN (UK), was climbing out of Chicago's runway 28R when the left inboard engine (RB211) emitted a series of bangs prompting the crew to stop the climb at 15,000 feet and return to Chicago reporting a #2 engine exceedance, the engine was experiencing . . .