What was found
AIRCRAFT WAS GROUNDED: ABNORMAL AIRFRAME VIBRATION STARTING AT ROTATION & CONTINUING UNTILL LANDING. VIBRATION FELT IN YOKE & PEDALS, INCREASED AS AIRSPEED INCREASED. QRH PROCEDURE FOR ELEVATOR TAB VIBRATION COMPLETED. DECESION MADE TO DIVERT TO ORD. EMERGENCY DECLARED & ACFT LANDED OVERWEIGHT WITHOUT INCIDENT. MAINT PFMD AIRFRAME VIBRATION CONDITIONAL INSP IAW AMM 05-51-67. ENG HIGH POWER PFMD WITH NO ABNORMALITIES. CONFIRMED WITH ORIGINAL. THE APU DOOR WAS IN OPEN POSITION ON ARRIVAL WITH APU CURENTLY UNDER MEL & CIRCUIT BREAKERS PULLED, COLLARD. DOOR WOULD MOST LIKELY HAVE BEEN OPENED PREVIOUSLY AT DCA RON DURING T/S CAUSING THE DOOR TO BE IN THE GND POSITION (45 DEGREES) RATHER THAN FLT POSITION (17 DEGREES). IAW AIRFRAME VIBE INSP STEP"C"(2)(F), THIS WOULD BE THE CAUSE OF THE VIBRATION. ENGINE DESK CONTACTED AND CONFIRMED NO OPERATIONAL LIMITS OR FURTHER REQUIRED INSPECTIONS FOR APU WINDMILLING ARE LISTED IN THE AMM. APU INLET DOOR/AUCTATOR WAS INSPECTED AND OPS CHKED WITH NO DEFECTS NOTED.