As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING A MAINTENANCE GROUND RUN OPERATION A HIGH LATERAL VIBRATION WAS ENCOUNTERED AND THE ACFT WAS SHUTDOWN. IT WAS DISCOVERED THAT WATER HAD ENTERED THE BLADE DURING A HEAVY RAINFALL THE NIGHT BEFORE. ATTEMPTS TO REMOVE THE WATER WERE UNSUCCESSFUL AND THE BLADE WAS REMOVED FROM THE ACFT FOR EVALUATION.