What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO CLT DUE TO THE FLIGHT CREW REPORTING THAT AT FL 280 SPEED 310 STARTED GETTING SEVERE VIBRATION FROM TAIL SECTION OF AIRCRAFT. FLIGHT ATTENDANTS REPORTED FROM ROW 15 TO REAR OF AIRCRAFT VIBRATION GOT WORSE AS YOU WALKED TO REAR OF AIRCRAFT. FLIGHT CREW RAN QRH PROCEDURE FOR ABNORMAL VIBRATION. SLOWED TO 280 KTS AND IT SUBSIDED. THE AIRCRAFT LANDED SAFELY AT CLT AND TAXIED TO THE GATE UNDER ITS OWN POWER. (OPEN)