MD80 CREW RETURNED TO THEIR DEP STATION AFTER HEARING AND FEELING A RUMBLING AFTER GEAR RETRACTION.

Narrative

AFTER THE ACFT WALKAROUND AND PREFLT CHKS WERE COMPLETED; A MECH CAME TO THE AIRPLANE TO RESOLVE 2 LOGBOOK ITEMS. WHILE CHKING A DISCREPANCY AND SITTING IN THE CAPT'S SEAT HE NOTICED THAT THE NOSE GEAR MECHANICAL DOWNLOCK INDICATOR WAS LOW. HE STATED THAT IT PROBABLY SHOULD BE ADJUSTED HIGHER. I SAID THAT I HAD SEEN THE INDICATORS IN THIS LOW OF A POS BEFORE. HE THEN CALLED ON THE PHONE FOR HELP AND FURTHER CLARIFICATION. I THEN BELIEVE THAT HE WENT OUTSIDE TO LOOK AT THE NOSE GEAR. DURING THIS TIME A MECH SUPVR CAME IN TO THE COCKPIT AND WE DISCUSSED THE INDICATOR. WE BOTH DECIDED THAT IT WAS ALL RIGHT TO GO THE WAY IT WAS. WITH MECHS OFF; AGENT CLOSED THE DOOR AND THEN WE TAXIED OUT FOR TKOF. AFTER TKOF AND WITH GEAR UP; THERE WAS A LOUD RUMBLE. ACCELERATING MADE THE NOISE LOUDER. WE SLOWED AND ASKED ORD DEP FOR A RETURN TO THE ARPT. JUST BEFORE THE TURN BACK WE PUT THE GEAR DOWN AND WITH A GEAR SAFE INDICATION WE LANDED. WE TAXIED BACK TO THE SAME GATE AND A MECH MET US. WE TOLD THIS NEW MECH OUR INDICATIONS. HE WENT OUTSIDE TO CHK AND FOUND THE NOSE GEAR FORWARD DOORS STILL OPEN. WE THEN KNEW THE FIRST MECH; WHO TOLD US ABOUT THE INDICATOR PIN; MUST HAVE OPENED THE DOORS AND THEN LEFT THEM OPEN WHILE LEAVING THE AIRPLANE.