B757 FLT CREW HAS A COCKPIT WINDOW PRESSURE LEAK.

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WE WERE LEVEL IN CRUISE FLT ABOUT 1 HR S OF ORD. AN EXTREMELY LOUD SQUEAL STARTED FROM THE AREA OF WINDOW 3R. WE CHKED PRESSURIZATION AND NOTHING HAD CHANGED WHICH WOULD CAUSE THE NOISE. WE ASKED FOR LOWER AND DSNDED TO FL320; STILL NO CHANGE. WE CONTINUED DSCNT TO FL240; STILL NO CHANGE. THE NOISE WAS SO LOUD THAT THE ATC CTLRS ASKED US TO TRY A DIFFERENT RADIO BECAUSE WE WERE BARELY READABLE ABOVE THE SQUEAL. WE TRIED THE OPPOSITE AUTO CTLR OF THE PRESSURIZATION; NO HELP. THE CABIN WAS NO LONGER AT MAX DIFFERENTIAL PRESSURE; BUT WE COULD BARELY COMMUNICATE WITH EACH OTHER AND THOUGHT OF A POSSIBLE DIVERT TO IND OR IIU. THE FO ASKED IF HE COULD TRY TO RAISE THE CABIN IN THE MANUAL MODE. I AGREED AND ASKED HIM TO MAKE SURE HE DIDN'T DROP THE MASKS. HE RAISED THE CABIN TO 4500 FT; WITH THE DIFFERENTIAL PRESSURE AT 6.8 PSI THE SQUEAL STOPPED AND WE CONTINUED TO ORD. I HAVE NEVER HEARD SUCH A LOUD; EAR PIERCING SQUEAL FROM A WINDOW IN FLT. IT WAS VERY DISTRACTING AND AS WE WERE COMING UP ON OUR 13TH HR OF DUTY TIME ON THE 4TH DAY OF A 4 DAY TRIP; WE WERE FORTUNATE TO BE ABLE TO QUIET THE SOUND AND CONTINUE TO ORD. I COMMEND THE FO ON A JOB WELL DONE.