What was found
MAIN CABIN DOOR ACOUSTIC SEAL GENERATES LOUD, HARMONIC VIBRATION WHEN DOOR IS OPERATED --PARTICULARLY WHEN IT IS BEING OPENED. THE DOOR JETTISONED OUTWARD UNDER CABIN PRESSURE. ALSO ACCOMPANIED BY CABIN VERTICAL SPEED RATES EXCEEDING 3000 FT./MIN. PRESSURIZATION CONTROL IN MANUAL MODE WITH KNOB AT MAX RATE. SWITCHED TO AUTO MODE AND ADJUSTED RATE TO MIDDLE OF RATE CLIMB AND RAN SYSTEM. SYSTEM OPERATES NORMALLY IAW MM 21-30-00. RAN AIRCRAFT WITH PRESSURIZATION IN AUTO MODE WITH NO NOISE NOTED. PILOTS CHECKLIST HAS WARNING STATEMENT THAT APU BLEED AIR SHOULD BE TURNED OFF OR A COCKPIT SIDE WINDOW OPENED PRIOR TO OPENING THE CABIN DOOR. VIBRATION WAS COMING FROM THE CABIN VENT DOOR NOT THE ACOUSTIC SEAL.