A B757 TAKES OFF WITH THE FO'S WINDOW NOT PROPERLY LATCHED. IT WAS PARTIALLY OPEN AND CREATED NOISE THAT INTERFERED WITH PLT TO PLT COM AND FLC TO ATC COM.

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RPTED TO ACFT AT APPROX AM15 FOR FLT FROM LAS TO MSY. AFTER COOLING DOWN THE COCKPIT USING AN OPEN WINDOW ON THE FO'S SIDE; I CLOSED THE WINDOW AND NOTICED THE WINDOW HANDLE IN THE CLOSED AND LOCKED POS AND WINDOW INDICATORS SHOWING CLOSED. NOTHING ABNORMAL WAS NOTED DURING WINDOW CLOSING PROCESS. DURING THE BEFORE START CHKS AND BEFORE TKOF CHKS; BOTH WINDOWS WERE CONFIRMED CLOSED AND LOCKED BY THE CAPT AND THE FO. DURING THE TKOF ROLL; COCKPIT NOISE INCREASED STEADILY. AT FIRST; BOTH CREW MEMBERS ATTRIBUTED THE NOISE TO AN INCREASE IN AIRFLOW THROUGH THE AIR CONDITIONING DUCTS; BUT AS ACFT SPD INCREASED; TO THE POINT WE COULD NOT COMMUNICATE; IT BECAME APPARENT THERE WAS A GAP IN THE WINDOW SEAL. BY THE TIME IT BECAME APPARENT AIRSPD WAS CLOSE TO V1 AND THE CAPT ELECTED TO CONTINUE THE TKOF. AFTER SAFELY GETTING AIRBORNE; I TURNED CTL OF THE ACFT OVER TO THE CAPT (FO MADE THE TKOF) AND ROTATED THE WINDOW HANDLE IN AN ATTEMPT TO FULLY CLOSE THE WINDOW. THE HANDLE JUST RATCHETED WHEN I ROTATED; INDICATING IT WAS AS FULL FORWARD AS IT COULD GO. AFTER SQUAWKING 7600 AND TURNING DOWNWIND VISUALLY; A SAFE LNDG WAS MADE TO RWY 25L AND FROM THERE WENT STRAIGHT TO THE GATE. MAINT MET THE ACFT AT THE GATE AND INDEED CONFIRMED THAT THE FO'S WINDOW WAS LOCKED AND SHOWING CLOSED WITH THE 'WINDOW NOT CLOSED' PLACARD OUT OF SIGHT; YET THERE WAS AN APPROX 1/2 INCH GAP IN THE TRAILING EDGE WINDOW SEAL. REAR TRACK BAR OF THE WINDOW WAS NOT QUITE PERPENDICULAR TO THE WINDOW SILL. FROM THE TIME THE WINDOW WAS CLOSED UNTIL MAINT INSPECTED THE WINDOW WAS APPROX 30 MINS AND DURING THAT TIME WINDOW LOCKING HANDLE WAS NOT TOUCHED AND WINDOW REMAINED IN THE SAME POS. AFTER MAINT CONFIRMED WINDOW INDICATING CLOSED AND LOCKED (WITH TRAILING EDGE GAP) WE WROTE THE WINDOW UP IN THE ACFT MAINT LOG. AFTER COMPLETING MAINT ON THE WINDOW; MAINT SIGNED THE DISCREPANCY OFF AND THE FLT CONTINUED TO MSY WITHOUT FURTHER PROB. SUPPLEMENTAL INFO FROM ACN 404677: WHEN WE BECAME AIRBORNE; THE NOISE BECAME SO LOUD THE FO AND I COULD NOT COMMUNICATE. I SOON REALIZED THAT WE ALSO COULD NOT HEAR THE TWR'S XMISSIONS. THE PROB AROSE BECAUSE THE WINDOW DID NOT CLOSE PROPERLY. I THINK THERE IS A DESIGN PROB WITH THE WINDOW THAT I BELIEVE CAUSED MANY PROBS IN THE PAST. IN THE FUTURE; I WILL RUN MY HAND DOWN THE AFT EDGE OF THE WINDOW AND ALSO CHK THE ACTUATION ARM POSITIONED PERPENDICULAR TO THE WINDOW. NEITHER OF THESE ACTIONS ARE IN OUR CHKLIST. ALSO IN THE FUTURE; IF I SUSPECT A WINDOW IS NOT SEALED PROPERLY; I WILL CONSIDER A HIGH SPD ABORT TO PREVENT THE SIT WE FOUND OURSELVES IN WHEN AIRBORNE. AS A HUMAN FACTORS CONSIDERATION; IT ADDS ANOTHER LAYER OF PROBS WHEN YOU CAN'T COMMUNICATE WITH YOUR FO. I BELIEVE PUTTING ON THE OXYGEN MASK AND THE OLD TYPE HEADSETS (SUPPLIED BY THE AIRLINE AND ON THE AIRPLANE) THAT FORM A SEAL AROUND THE EAR COULD HAVE AIDED US IN COMMUNICATING INSIDE AND OUTSIDE THE ACFT. I WAS GRATEFUL FOR THE HUMAN FACTORS TOOL OF A DEP. DURING THE DEP BRIEFING WE HAD NOTED A NOTAM THAT CHANGED THE DEP FOR RWY 25R BECAUSE BLD VOR WAS OTS. THE EXECUTION OF THIS PROC WAS MY INITIAL ACTION AFTER OUR DEP.