Narrative:

Sat on runway in ZZZ for 3 hours waiting for thunderstorms to go over. Lightning hit the tower and no doppler radar, also striking a runway which had to be closed due to holes and debris from strike. When we finally takeoff for dfw (not sure of the time) we were still almost full of passenger and also heavy in fuel. Immediately after takeoff the #2 flight attendant called the cockpit and informed them of a load almost explosion type noise and a distinct vibration and rattling near the left engine. (I wanted to add that flight attendant #2 also stated the floor under jump seat raised some.) the co-pilot came back and agreed that there was a problem, was unsure what it could be. I went back and observed the sound and felt the distant vibration and indicated to the cockpit that I had never heard a noise on the aircraft like that in 30 years. The captain decided to make an unscheduled landing at iad. When we landed we were far from the terminal and many fire engines were there to greet us. Several firemen stood in front of aircraft observing engines and the whole fuselage until we were permitted to return to gate. It was the door by the engine, which is opened to fit hose to flush the lavs. Apparently it had no been secured properly and when we takeoff the thrust of takeoff forced air into it and caused the explosion sound and the slight lift of the floor under flight attendant #2 jumpseat. The rattling sound and vibration was the flapping of the door.

Google
 

Original NASA ASRS Text

Title: A MD80 WAS DISPATCHED IN NON COMPLIANCE WITH THE LAV SVC DOOR NOT LATCHED RESULTING IN NOISE AND VIBRATION. FLT DIVERTED FOR INSPECTION.

Narrative: SAT ON RWY IN ZZZ FOR 3 HOURS WAITING FOR TSTMS TO GO OVER. LIGHTNING HIT THE TWR AND NO DOPPLER RADAR, ALSO STRIKING A RWY WHICH HAD TO BE CLOSED DUE TO HOLES AND DEBRIS FROM STRIKE. WHEN WE FINALLY TKOF FOR DFW (NOT SURE OF THE TIME) WE WERE STILL ALMOST FULL OF PAX AND ALSO HVY IN FUEL. IMMEDIATELY AFTER TKOF THE #2 FLT ATTENDANT CALLED THE COCKPIT AND INFORMED THEM OF A LOAD ALMOST EXPLOSION TYPE NOISE AND A DISTINCT VIBRATION AND RATTLING NEAR THE LEFT ENG. (I WANTED TO ADD THAT FLT ATTENDANT #2 ALSO STATED THE FLOOR UNDER JUMP SEAT RAISED SOME.) THE CO-PLT CAME BACK AND AGREED THAT THERE WAS A PROBLEM, WAS UNSURE WHAT IT COULD BE. I WENT BACK AND OBSERVED THE SOUND AND FELT THE DISTANT VIBRATION AND INDICATED TO THE COCKPIT THAT I HAD NEVER HEARD A NOISE ON THE ACFT LIKE THAT IN 30 YEARS. THE CAPT DECIDED TO MAKE AN UNSCHEDULED LANDING AT IAD. WHEN WE LANDED WE WERE FAR FROM THE TERMINAL AND MANY FIRE ENGS WERE THERE TO GREET US. SEVERAL FIREMEN STOOD IN FRONT OF ACFT OBSERVING ENGS AND THE WHOLE FUSELAGE UNTIL WE WERE PERMITTED TO RETURN TO GATE. IT WAS THE DOOR BY THE ENG, WHICH IS OPENED TO FIT HOSE TO FLUSH THE LAVS. APPARENTLY IT HAD NO BEEN SECURED PROPERLY AND WHEN WE TKOF THE THRUST OF TKOF FORCED AIR INTO IT AND CAUSED THE EXPLOSION SOUND AND THE SLIGHT LIFT OF THE FLOOR UNDER FLT ATTENDANT #2 JUMPSEAT. THE RATTLING SOUND AND VIBRATION WAS THE FLAPPING OF THE DOOR.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.