AN EMB120 IS MANDATED TO RETURN LAND AFTER SEVERE VIBRATIONS ARE FELT AFTER TKOF FROM ATL; GA.

Narrative

SHORTLY AFTER XA00; FLT DEPARTED ATL BOUND FOR MCN. IMMEDIATELY AFTER TKOF; VERY STRONG AND LOUD VIBRATIONS FORCED US TO RETURN FOR IMMEDIATE LNDG. IT WAS FOUND THAT THE CARGO DOOR HAD BEEN LEFT OPEN. SHORTLY AFTER BOARDING; PRIOR TO THE FLT; I CHKED THE MAINT LOG AND DEFERRAL LOG. WE FOUND THAT THE CARGO DOOR INDICATION LIGHT HAD BEEN DEFERRED. AS WAS CUSTOMARY; I ADVISED THE FO SHORTLY BEFORE DEP TO INSPECT THE FUEL PANEL DOORS; BUT ALSO TO INCLUDE THE CARGO DOOR IN HIS LAST MIN CHK. THIS WAS DONE APPROX 5 MINS BEFORE DEP. ABOUT 3 MINS BEFORE DEP; THE RAMP AGENT WHO LOADED OUR AIRPLANE HANDED US THE PAPERWORK TO COMPLETE THE LOAD MANIFEST FORM. IT SEEMED REASONABLE TO ASSUME THE PERSON LOADING OUR AIRPLANE WOULD ALSO SERVE AS A 'DOUBLE-CHK' TO THE STATUS OF OUR CARGO DOOR; AND THAT AFTER ALL BAGS HAD BEEN LOADED AND THE QUANTITY OF BAGS VERIFIED; THAT THE RAMP AGENT WOULD MAKE SURE THE CARGO DOOR WAS CLOSED. THE FO AGAIN VERBALLY VERIFIED TO ME THAT THE CARGO DOOR WAS CLOSED IMMEDIATELY PRIOR TO ENG START. WE LATER DISCOVERED THAT RAMP AGENT'S ORIGINAL COUNT OF THE BAGS WAS TERRIBLY INACCURATE. STATING; ORIGINALLY; THAT WE HAD 5 CHKED BAGS ONLY; WE FOUND THAT WE TRULY HAD 2 CHKED BAGS; 1 CARRY-ON BAG; 2 LBS OF CARGO AND 5 LBS OF COMPANY MAIL. THIS POINTED TO 1 OF 2 POSSIBILITIES. EITHER EXTRA CARGO WAS ADDED WITHOUT THE RAMP AGENT'S KNOWLEDGE; OR THE RAMPER'S IRRESPONSIBILITY AND LACK OF ACCURACY IN COUNTING BAGS LED TO A LAST MIN BREAKDOWN IN SAFETY. WE LATER CONTINUED THE FLT TO MCN WITH NO FURTHER PROBS.