What was found
AFTER TAKEOFF, A LOUD NOISE FROM NOSE GEAR, VIBRATION IN BOTH RUDDER PEDDLES. NOISE AND VIBRATION CONTINUED UP TO 230 KNOTS. BELOW 160 KNOTS NOISE QUIT. QRH COMPLETED FOR STALL FAIL. EMERGENCY WAS DECLARED AND AIRCRAFT RETURNED TO DEPARTURE. FOUND O2 SERVICE DOOR AND CAP OPEN. NO DAMAGE WAS FOUND TO DOOR OR SURROUNDING STRUCTURE CAP ON PANEL. CLOSED DOOR, OPS CHECK OF STALL PROTECTION SYS GOOD IAW AMM 27-35-00. OK FOR SERVICE.