As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CAPTAIN REPORTED AN UNUSUAL VIBRATION IN AFT OF AIRCRAFT AFTER TAKE OFF. THE AIRCRAFT WAS GROUNDED. MAINTENANCE FOUND THE AFT POTABLE WATER TANK VIBRATING WHILE THE ENGINES ARE RUNNING ABOVE IDLE DUE TO TURNBUCKLE CABLE ASSEMBLY LOOSE AND THE FOAM BLOCKS THAT SUPPORT THE TANK WITH THE TEE FITTING WERE OFF THE TANK CAUSUNG THE TANK TO VIBRATE. REINSTALLED THE FOAM BLOCKS AND ADJUSTED THE SUPPORT RING ASSEMBLY IAW AMM 38-11-01.