A DC8 FO EXTERNAL WINDOW RELEASE WAS NOT CLOSED PRIOR TO FLT CAUSING A LOUD NOISE AND PRESSURIZATION PROBLEMS.

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AFTER TKOF WHEN THE FIRST PACK WAS TURNED ON A LOUD AIR RUSH WAS HEARD FROM THE FO'S SLIDING WINDOW AREA. WE INITIALLY THOUGHT THE WINDOW WAS NOT COMPLETELY CLOSED AND TRIED TO SEAT IT AS WE CONTINUED TO PRESSURIZE; THE WINDOW WAS OPENED ON THE GND WHEN WE WERE IN THE BLOCKS TO ATTEMPT TO GET THE MECH'S ATTN WHEN WE LOST GND PWR DURING ENG START. THE SOUND CONTINUED TO INCREASE AS WE CONTINUED TO CLB AND THE PRESSURE DIFFERENTIAL INCREASED. WE WERE HAVING SOME DIFFICULTY MAINTAINING PRESSURIZATION. AS WE CLBED; WE HAD TO INCREASE THE FLOW BELOW FL180. NOT BEING ABLE TO CTL THE NOISE AND DIFFICULTY MAINTAINING PRESSURE; WE DSNDED TO 10000 FT AND LOWERED THE DIFFERENTIAL PRESSURE AND THE NOISE SUBSIDED SOMEWHAT. WE CALLED FLT CTL AND MAINT CTL AND INFORMED THEM OF OUR DECISION TO CONTINUE TO ZZZ1 AT 10000 FT. MAINT THOUGHT IT MIGHT BE DUCT WORK UNDER THE FO'S FLOOR THAT WE WERE HEARING; WE THEN CYCLED THE PACKS ON AND OFF TO SEE IF THE NOISE LEVEL CHANGED; IT DID NOT SIGNIFICANTLY CHANGE. AS WE DSNDED INTO ZZZ1 AND THE PRESSURE DIFFERENTIAL DECREASED THE NOISE DECREASED AS WELL. DURING A POSTFLT INSPECTION; WE FOUND THE FO'S EMER WINDOW RELEASE DOOR WAS OPEN. WE COULD NOT UNDERSTAND HOW WE WOULD MISS THIS ON PREFLT -- ESPECIALLY SINCE THE FO HAD THE WINDOW OPEN TRYING TO GET THE ATTN OF THE MECH WHEN THE GND PWR FAILED DURING ENG START. WE SURMISED THAT WHEN HE WAS TRYING TO GET THE MECH'S ATTN AND SLAPPING HIS HAND AGAINST THE SIDE OF THE ACFT; HE INADVERTENTLY OPENED THE EMER ACCESS DOOR WITHOUT REALIZING IT.