As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, THE CREW OBSERVED SPARKS EXITING FROM THE REAR OF THE NR 3 ENGINE EVERY 15 TO 20 SECONDS. ALL ENGINE INSTRUMENTS INDICATED NORMAL AND NO VIBRATION WAS NOTICED. THE FLIGHT RETURNED TO BOS. MAINTENANCE FOUND MOLTEN METAL ON THE BLEED SCREEN AND TAILPIPE. THE AIRCRAFT WAS REMOVED FROM SERVICE AND FERRIED ON TWO ENGINES TO MSP.