As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED DUE TO: DURING CLIMBOUT THERE WERE CONTINUOUS AIRFRAME VIBRATIONS AS AIRCRAFT ACCELERATED TO CLIMB SPEED. AIRCRAFT EMERGENCY LANDED SAFELY AT PHL. UPON LANDING APU INLET DOOR WAS FULLY OPEN WHILE OM MEL 49-51. APU INLET SHOULD HAVE BEEN FULLY CLOSED, PER MEL 49-51. CLOSED AND VERFIED APU INLET PER MEL 49-51. NO FURTHER DAMAGE NOTED.