What was found
VERY HEAVY VIBRATION FROM LEFT ENGINE DURING ENGINE START UP & TAXI. DID ENGINE RUN UP & IT GOT WORSE. NO BAD ENGINE INDICATIONS ON EICAS. A/C RETURNED TO BLOCKS (BTB). AIRCRAFT FREIGHT LOAD IS LIGHT AT 15,500. PERFORMED ENGINE IDLE RUN AND BREAK AWAY POWER RUN UP OF BOTH ENGINES IAW AMM 71-00-00, WITH EEC'S ON AND OFF. ALL INDICATIONS NORMAL, NO VIBRATIONS NOTED. PERFORMED GVI INSP WITH NO DEFECTS NOTED. A/C OK FOR SVC. A/C REMAINS NO ETOPS.