FLC OF B757-200 HEARS HIGHER THAN USUAL NOISE AT TKOF. THE NOISE BECOMES LOUDER AND MORE DISTRACTING ONCE AIRBORNE. INSPECTION FINDS THE FO WINDOW HAS A GAP AT THE BACKSIDE. RETURN LAND EXECUTED.

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ALL GND OPS/PROCS HAD BEEN NORMAL UP TO ABOUT 80 KTS ON TKOF ROLL WITH THE FO PERFORMING PF DUTIES. THE COCKPIT NOISE LEVEL BECAME HIGHER THAN USUAL BUT SEEMED TO BE ASSOCIATED WITH HIGH AIR FLOW FROM THE AIR CONDITIONING PACKS. ONCE AIRBORNE; THE NOISE LEVEL BECAME EXCESSIVE AND PREVENTED NORMAL COMS BTWN FLT DECK PERSONNEL. AT THIS POINT; THE CAPT INFORMED THE TWR IT WOULD BE NECESSARY TO RETURN TO PHX. THE ACFT WAS SLOWED TO 180 KTS WITH A CLB TO 7000 FT MSL. WHILE ON DOWNWIND; A CURSORY CHK OF THE FO'S SLIDING SIDE WINDOW REVEALED A SLIGHT GAP BTWN THE BACK SIDE OF THE WINDOW AND THE WINDOW FRAME. A VISUAL PATTERN WAS FLOWN. LNDG AND TAXI TO THE GATE WAS NORMAL. AFTER ENG SHUTDOWN; MAINT PERSONNEL CAME ON BOARD AND INSPECTED THE FO'S SIDE WINDOW. EVEN THOUGH STILL INDICATING CLOSED AND LOCKED; THE BACK PORTION OF THE WINDOW HAD OPENED SLIGHTLY DURING TKOF ROLL WHICH RESULTED IN THE EXCESSIVE NOISE AND LOSS OF PRESSURIZATION. THE WINDOW WAS CYCLED SEVERAL TIMES AND CLRED BY MAINT. POSSIBLE CORRECTIVE ACTION: LCL -- IF FLT DECK WINDOWS ARE OPENED WHILE THE ACFT IS RELEASED TO MAINT; A MAINT SUPVR/QUALIFIED INDIVIDUAL SHOULD VERIFY PROPER WINDOW CLOSURE OR LEAVE THE WINDOW OPEN (WX PERMITTING) FOR THE ACFT'S RELEASE TO OPS. ACFT MANUFACTURER -- CHANGE THE DESIGN OF THE WINDOW LOCKING HANDLE SO THAT IT CANNOT BE MOVED INTO THE LOCKING SLOT UNTIL THE WINDOW HAS BEEN CRANKED INTO THE FULLY CLOSED POS. IN ADDITION; THE WINDOW 'CLOSED' INDICATOR SHOULD REMAIN OBSCURED UNTIL THE WINDOW IS COMPLETELY CLOSED AND FLUSH. IN THE EVENT THIS SIT ARISES AGAIN; THERE SHOULD BE AN APPROVED PROC WHICH WOULD ALLOW FULL WINDOW CLOSURE WHILE AIRBORNE WITHOUT JEOPARDIZING THE SAFETY OF THE CREW OR PAX.