What was found
DURING CLIMB OUT STARTING AT 16000 FT AND ABOVE AROUND 265 KIAS, A VIBRATION FELT UNDER FEET, COMING FROM WHAT SUSPECT IS NLG DOOR. VIBRATION FELT ALL THE WAY UP TO AROUND FL250 & OUR INDICATED AIRSPEED STARTED TO DECREASE AT ALTITUDE, THEN IT STOPPED. WHILE WE FELT VIBRATION, DECREASED ENG RPMS TO MAKE SURE IT WAS NOT COMING FROM ENGS. NO CHANGE IN VIBRATION. DECREASED AIRSPEED BELOW 265 & IT STOPPED, & STARTED AGAIN AS SOON AS INCREASED ABOVE 265. INSPECTION FOUND THE NNLG DOOR WAS NOT CLOSING ALL THE WAY PROPERLY. ACCOMPLISHED VIBRATION TROUBLESHOOTING IAW TASK CARD TC-C68-53-1001, INCLUDING ENGINE RUN CHECKS, ENG VISUAL INSPS, EXTERIOR SERVICE DOORS, BELLY ACCESS PANELS, ALL NLG & MLG DOORS, PRIMARY & SECONDARY FLT CONTROLS. VERIFIED AVIONICS COOLING FAN IS NOT SOURCE. CONCENTRATED ON FWD FUSE, RADOME, NOSE AVIONICS DOORS & NLG DOORS. ACCOMPLISHED GEAR RETRACTION & VERIFIED ALL NLG DOORS ARE TIGHT & AERODYNAMICALLY SMOOTH. FOUND RT NOSE AVIONICS DOOR SEAL LOOSE & DETACHED.