FLC OF MDT RETURNED AND LANDED DUE TO BANGING NOISE ON THE SIDE OF THE ACFT DURING CLBOUT.

Narrative

SHORTLY AFTER DEP OF ACR FLT FROM DTW-BMI; WE NOTICED A FLUTTERING SOUND COMING FROM THE R SIDE OF THE FUSELAGE. AS WE CONTINUED THE CLB THE SOUND WAS INTERMITTENT; AND EVENTUALLY BECAME LOUDER AND MORE REGULAR RESEMBLING THE SOUND OF POPCORN POPPING. ALL INSTS AND GAUGES ARE SHOWING NORMAL INDICATIONS. WE DETERMINED THAT THE NOISE WAS COMING FROM THE FORWARD SECTION OF THE R FUSELAGE WHERE THE LAVATORY SVC DOOR IS LOCATED. THE DECISION WAS MADE TO RETURN TO DTW AS A PRECAUTIONARY MEASURE. NO SPECIAL ASSISTANCE WAS REQUESTED FROM DTW ATC. AFTER LNDG AND RETURNING TO COMPANY'S GATE; RAMP PERSONNEL POINTED TO THE DOOR AND RESECURED EITHER AN OPEN LATCH OR DOOR. COMPANY MAINT PROCS WERE COMPLIED WITH AND AFTER RERELEASE FROM COMPANY DISPATCH THE FLT RESUMED. BEFORE ORIGINAL DEP; I CONDUCTED A PREFLT WALKAROUND OF THE ACFT AFTER RETURNING FROM COMPANY'S OPS APPROX 25-30 MINS BEFORE DEP TIME. I OBSERVED THE ACFT'S SVC DOORS TO ALL BE CLOSED. I THEN ENTERED THE ACFT TO COMPLETE PAPERWORK AND OTHER PREFLT DUTIES. I WAS UNAWARE OF ANY SUBSEQUENT SVCING BEING CONDUCTED. I BELIEVE THIS HAPPENED BECAUSE OF INADVERTENT OPENING IN FLT OR POSSIBLY IMPROPER SECURING OF THE DOOR ON THE GND. I BELIEVE A POSSIBLE GND SECURING PROB COULD BE AVOIDED BY GREATER COM BTWN GND PERSONNEL AND CREW WHEN SVCING OCCURS.