As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED DUE TO: LEVELING OFF AT FL310 NR 1 ENGINE N1 DIPPED AND SURGED AT LEAST 3 TIMES. THIS WAS ACCOMPANIED BY A BANGING NOISE HEARD IN THE COCKTAIL. FLIGHT ATTENDANT REPORTED SEEING SPARKS COMING FROM THE NR 1 ENGINE AS WELL. ALSO HEARD AN AUDIBLE BANGING SOUND, AND REPORTED FEELING THE CABIN FLOOR VIBRATE. AIRCRAFT AIR INTERRUPTED BACK TO AND LANDED SAFELY AT ORD. (OPEN).