What was found
FLIGHT 6393 - CVG-BNA - DURING CRUISE AT 16,000 FT & APPROX 310 KIAS, ACFT STARTED SHAKING & FLUTTERING WITHOUT WARNING. AUTOPILOT ENGAGED AT THE TIME. LATER, MASTER WARNING LIGHT & CHIME SOUNDED, WITH MESSAGES -PITCH TRIM MAIN INOP- &-PITCH TRIM BACKUP INOP-. AT THIS TIME, AUTOPILOT DISCONNECTED USING QUICK DISCONNECT BUTTON. ACFT VERY HARD TO CONTROL ABOUT PITCH AXIS, REQUIRING EXCESSIVE FORCE INPUTS IN ORDER TO MAINTAIN CONTROLLED LEVEL FLT. DECLARED AN EMERGENCY & REQUESTED DIRECT SDF. AIRSPEED REDUCED, & WHILE FIGHTING WITH VERTICAL CONTROL, BEGAN LOSING ALTITUDE IN A SHALLOW DESCENT. VERY STRONG BACK PRESSURE HAD TO BE KEPT ON CONTROLS TO KEEP THE DESCENT SHALLOW. PITCH TRIM INDICATIONS SHOWING -