A B757 CREW RPTS ENG MALFUNCTION DURING CLBOUT. AFTER CONSULTING WITH MAINT; AN EMER WAS DECLARED AND THEY RETURNED TO DEP ARPT.

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DURING CLBOUT; I FELT A SHUDDER AND THE ACFT ROLL; SIMILAR TO A WAKE TURB ENCOUNTER. A FEW MOMENTS LATER IT HAPPENED AGAIN AND THE FO RPTED THE L ENG INSTS WERE ERRATIC. I THEN OBSERVED FLUCTUATING EPR; AND N1 AND EGT ON THE ENG. FLT ATTENDANTS ALSO RPTED AN ABNORMAL GRINDING SOUND IN THE CABIN; WHICH THEY FELT CAME FROM THE ENG. WE GOT A CLRNC TO LEVEL OFF AND ACCOMPLISHED THE APPROPRIATE QRC CHKLIST AND REF ITEMS. WITH THE CHKLIST COMPLETED DISPATCH WAS NOTIFIED. HE PROVIDED A QUICK PHONE PATCH WITH MAINT. SINCE THE ENG OPERATED SATISFACTORILY AT A REDUCED PWR SETTING IT WAS DECIDED TO LEAVE IT RUNNING AND RETURN. WE DECLARED AN EMER AND RECEIVED CLRNC BACK TO ZZZ; THE NEAREST AND SAFEST ARPT. I BRIEFED THE PURSER AND MADE A CABIN ANNOUNCEMENT TO CALM THE PAX AS THEY HAD HEARD THE UNUSUAL NOISE AS WELL. A NORMAL 2-ENG APCH WAS PLANNED. HOWEVER; WE REVIEWED THE SINGLE ENG PROCS IN THE EVENT THAT THE PROB ENG SUBSEQUENTLY FAILED. AN OVERWT LNDG WAS ACCOMPLISHED. LNDG AND TAXI IN WERE NORMAL. MAINT WAS DEBRIEFED AT THE GATE. COCKPIT CRM WAS EXCELLENT.