FLT CREW OF B777-200 ENCOUNTER EXCESSIVE VIBRATION LEVELS FROM L ENG SHORTLY AFTER DEP OUT OF JFK. DUMP FUEL AND RETURN FOR MAINT.

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PREFLT AND FLT PLANNING WERE NORMAL FOR THIS FLT. ONE MEL DEFERRAL WAS GIVEN FOR 'STAT' MESSAGE 'LAVATORY GALLEY FAN R.' TKOF WAS NORMAL WITH NO ADVERSE NOISE OR CTL PROBS. CLBOUT WAS NORMAL WITH THE EXCEPTION THAT AT APPROX 10000 FT MSL; FO COMMENTED THAT '...THIS IS A LOUD AIRPLANE IN THE COCKPIT.' THIS WAS AN AIR NOISE; NOT ENG; AND CREW'S THINKING WAS THAT THE MEL MIGHT BE RELATED. LEVELOFF WAS AT FL330. BOTH PLTS NOTICED THAT L ENG VIBRATION WAS HIGHER (2.8) THAN R (0.7). WITHIN 15 MINS; PURSER CALLED COCKPIT TO ADVISE THAT THERE WAS A LOUD VIBRATION ON L ENG SIDE OF ACFT. AT THIS POINT; WE WERE 5-15 MINS PAST TUSKY. COCKPIT CREW IMMEDIATELY SENT ACARS MESSAGE TO MAINT ADVISING OF L ENG VIBRATION AND NOISE IN CABIN. AFTER APPROX 7 MINS; WE RECEIVED A MESSAGE FROM MAINT: 'VIBES LOOK EXCESSIVE FROM OUR POINT OF VIEW; I SUGGEST RETURNING TO JFK. CONTACT ME VIA COMRDO FOR ADDITIONAL INFO.' WE ATTEMPTED AND ACCOMPLISHED SATPHONE CALL TO MAINT. AFTER DISCUSSION WITH MAINT ABOUT ENG INDICATIONS AND INFO THAT ONLY HE HAD ACCESS TO; WE ASKED HIM IF HE THOUGHT WE SHOULD LAND EARLIER THAN JFK; AND HIS RESPONSE WAS NO; THEY WANTED US TO RETURN TO JFK. WE THEN ASKED HIM TO PATCH US THROUGH TO DISPATCH AND HE DID. AFTER DISCUSSION WITH DISPATCH; WE RECEIVED AN ATC CLRNC BACK TO JFK VIA TUSKY. DSCNT ENRTE TO JFK CREW; DISPATCH AND COMPANY MAINT DISCUSSED OPTIONS OF LNDG OVERWT OR DUMPING FUEL. RWY LIMIT WTS; LNDG DATA; WX WERE ALL CHKED AND CREW DECISION WAS TO JETTISON FUEL TO 52000 LBS IN DSCNT; SO THAT LNDG WT WOULD BE 445000 LBS OR LESS. DSCNT WAS NORMAL AND A FLAPS 30 DEG LNDG WAS ACCOMPLISHED ON RWY 31R JFK. SUPPLEMENTAL INFO FROM ACN 615381: PURSER ADVISED THAT HE HAD TO RELOCATE A PAX TO A DIFFERENT SEAT DUE TO VIBRATION OF THE PAX SEAT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE L ENG VIBRATION INDICATION WAS OUT OF LIMITS AND THE NOISE IN THE FORWARD CABIN WAS ANNOYING AND DISTRACTING. THE RPTR SAID THE ENG WAS NOT SHUT DOWN. THE RPTR STATED THAT FUEL WAS DUMPED TO MAX LNDG WT AND LNDG WAS NORMAL. THE RPTR SAID THE ENG WAS CHANGED IN THE DIVERTING STATION.