A B757 FLT CREW HAD TO DEAL WITH EXCESSIVE NOISE FROM THE FO'S WINDOW WHEN IT PARTIALLY OPENED IN FLT. THEY RETURNED TO DEP ARPT AND FOUND A DISCARDED PIECE OF A BOLT IN THE WINDOW CHANNEL.

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ON TKOF ROLL AT APPROX 80-100 KIAS NOISE LEVEL IN COCKPIT NOTICEABLY HIGHER THAN NORMAL FROM BACK OF FO'S SEAT. CONTINUED THE TKOF; NOISE CONTINUED TO INCREASE. ON INITIAL LEVELOFF AT 3000 FT DECIBEL LEVEL REACHED A POINT WHERE VERBAL COM IMPOSSIBLE. FO ATTEMPTED TO RESEAT WINDOW WHERE NOISE/AIR LEAK WAS EMANATING AND WAS UNABLE. I SIGNALED FOR RETURN TO ZZZ AND FO WEARING A NOISE REDUCTION HEADSET COMMUNICATED WITH APCH AND RECEIVED VECTORS TO LOC AT ZZZ. NOTIFIED FLT ATTENDANTS AND PAX OF SIT. FLT ATTENDANTS COULD HEAR THE NOISE AS WELL AS THE FIRST FEW ROWS OF FIRST CLASS AND COULD BARELY HEAR INTERPHONE. PA ANNOUNCEMENT WAS RECALLED AS 'LOUD' AND CLR. NORMAL APCH AND OVERWT LNDG AT APPROX 202000 LBS. AFTER TAXI IN FO NOTICED DOOR LOCK ARM ABOUT 20 DEGS REAR OF NORMAL PERPENDICULAR LOCKED CONDITION. WINDOW WAS CHKED IN THE NORMAL LOCKED STATUS DURING CHKLIST AND TAXI OUT. MAINT CHKED OP AT GATE AND WINDOW CLOSED LOCKED NORMALLY. HOWEVER; THEY DID FIND A SMALL BOLT IN THE WINDOW CHANNEL. THEY COULD NOT DETERMINE IF THIS CAUSED THE WINDOW TO BECOME PARTIALLY UNLOCKED. OVERWT INSPECTION SHOWED NO ABNORMAL INDICATIONS AND AFTER NOTIFYING DISPATCH AND MAINT WE RECONFIGURED THE ACFT AND DEPARTED APPROX 45 MINS LATER FOR ZZZ. NO RECURRENCE AND ROUTINE FLT. SUPPLEMENTAL INFO FROM ACN 789238: BY ROTATION IT WAS DEAFENING LOUD AND AIR WAS BLOWING THROUGH A GAP WHERE THE AFT PORTION OF THE WINDOW WAS PARTIALLY OPEN; THE ARM WHICH IS SUPPOSED TO BE AT 90 DEGS WAS OFF BY ABOUT 15 DEGS. THE WINDOW HAD BEEN PROPERLY RATCHETED CLOSED AND INDICATED LOCKED WITH THE ARM 90 DEGS PRIOR TO TKOF. CHKLISTS WERE COMMUNICATED VIA HAND SIGNALS AND SCREAMING. UPON INSPECTION AFTER LNDG THE SHEARED HEAD OF AN OLD RUSTY BOLT OR NUT WAS FOUND IN THE TRACK AND REMOVED; AND THE WINDOW OPERATED NORMALLY.