A320 FLIGHT CREW REJECTS TAKEOFF WHEN N1 VIBRATION EXCEEDS LIMITS. CREW HAD PREVIOUS EXPERIENCE WITH THIS AIRCRAFT BUT MAINTENANCE COULD FIND NOTHING WRONG.

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ACFT X WAS THE ACFT WE HAD DIVERTED TO ZZZ IN FROM ZZZ1 THE NIGHT BEFORE. WE HAD AT THAT TIME ENCOUNTERED HIGH N1 VIBRATION (6.1 ON ENG #2) DURING OUR GAR AND SUBSEQUENT DIVERSION. WHILE IT MAY HAVE BEEN THE ICING CONDITIONS WE FLEW IN; WE WERE NOT SURE; AND ENG #1; WHICH FLEW IN THE SAME CONDITIONS; WAS FINE. WE WROTE IT UP AND THE NEXT DAY FOUND THAT ENG #2 HAD BEEN INSPECTED; RUN; AND NOTHING FOUND AMISS. DURING PREFLT INSPECTION; THE FO SPUN THE ENG #2 N1 FAN; AND NOTICED A GRINDING SOUND. I WENT DOWN AND SPUN THE ENG #2 N1; AND HEARD THE SAME SOUND; LIKE THAT OF GRAVEL IN A ROTATING TIN CAN. WE WROTE IT UP. I SUSPECTED ICE CHUNKS LOOSE IN THE ENG #2 NOSE CONE; THOUGH I HAD NO IDEA HOW THAT COULD HAPPEN. WITH MAINT ABOARD; WE RAN THE ENG; AT LOW PWR; ANTI-ICE ON; UNTIL IT WARMED UP. NO UNUSUAL OR ABNORMAL INDICATIONS WERE NOTED; BUT AS THE N1 SPOOLED DOWN; MAINT NOTICED ICE PELLETS BEING SLUNG FROM THE AREA OF THE NOSE CONE. I WENT DOWN SAW THEM TOO. THE ENG WAS AGAIN SIGNED OFF; AND WE PUSHED BACK FOR ZZZ. TKOF WAS PLANNED FOR FLEX OF 37 DEGS; FLAPS 2 DEGS; ENG ANTI-ICE OFF. WHEN THE FO STOOD THE THROTTLES UP AND THE ENGS BEGAN TO SPOOL; ENG #2 VIBRATION QUICKLY REACHED 7. I ELECTED TO REJECT THE TKOF AND DID SO ACCORDING TO PROCS. THRUST WAS NEVER FULLY SET; AND THE SPD WAS VERY LOW; I WOULD ESTIMATE NO MORE THAN 50 KTS; THOUGH THAT'S A GUESS. WE EXITED THE RWY AND RETURNED TO THE GATE; LEAVING BOTH ENGS RUNNING. ENG #2 VIBRATION WAS NORMAL AT IDLE ON THE GND. NO EMER WAS DECLARED. FLT WAS CANCELED.