What was found
IN FLIGHT CLIMBING THROUGH FL 300, ANTI ICE OFF, MACH .78, AIRFRAME VIBRATION WAS FELT. MORE SIGNIFICANT IN THE CABIN AND LAVATORY THAN COCKPIT. VIBRATION WAS INTERMITTENT. WAS NOT RELATED TO THROTTLE POSITION BUT OCCURRED AT SPEEDS AT OR ABOVE MACH .78. CORRECTIVE ACTION, PERFORMED ENGINE VIB SURVEYS ON BOTH ENGINE NO DEFECTS NOTED. COULD DUPLICATE VIBRATION ON GROUND ON 05-13-2019. PERFORMED VISUAL INSPECTION OF A/C AND FOUND LT MLG DOOR SEAL DETERIORATED. JACKED A/C AND RETRACTED GEAR AND CHECKED ALL GEAR DOORS ALL GOOD. REMOVED AND REPLACED LT MLG DOOR SEALS ALONG WITH ALL FOUR SLAT BUZZ STRIPS ON EACH WING.