B767 CAPT REPORTS AIR LEAK ON CAPT SLIDING WINDOW THAT PRODUCES PIERCING WHISTLE. ANOMALY HAS BEEN WRITTEN UP 4 TIMES PREVIOUSLY AND CAPT ELECTS TO RETURN TO LAND.

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THIS ACFT HAD BEEN SUPPOSEDLY 'FIXED;' ON 4 DIFFERENT OCCASIONS PREVIOUSLY TO OUR BEING ASSIGNED TO FLY IT FROM ZZZ TO ZZZ1. THE CAPT'S SLIDING WINDOW OBVIOUSLY HAD A MAJOR PROB WHICH CONTINUED TO REPEAT ITSELF 4 TIMES PREVIOUS. WITH SOMETHING AS SERIOUS AS A WINDOW AND ITS RELATED NOISE; IT SEEMS QUITE SURPRISING THAT THE ACFT WOULD CONTINUE TO REMAIN IN SVC FOR A REOCCURRING PROB ON 4 DIFFERENT OCCASIONS BEFORE WE BECAME #5. WE WERE ABOUT AN HR AND A HALF OUT FROM ZZZZ WHEN THE CAPT'S WINDOW SUDDENLY MADE A PIERCING WHISTLE THAT ACTUALLY HURT MY EARS. IT CONTINUED UNABATED. IT WAS SO SEVERE; THERE MAY HAVE EVEN BEEN HEARING DAMAGE AS A RESULT OF THIS. IT WAS VIRTUALLY IMPOSSIBLE TO HEAR RADIO XMISSIONS AND FOCUS ON VARIOUS TASKS IN THE COCKPIT INCLUDING EVEN FLYING THE AIRPLANE WITH THE CONCENTRATION REQUIRED IN A NORMAL SITUATION GIVEN THE LEVEL OF NOISE. I ELECTED TO REVERSE COURSE AND RETURN TO ZZZZ SEEING HOW IT WOULD BE IMPOSSIBLE TO CONTINUE GIVEN THE SERIOUSNESS OF THE PROB. OUR EXPERT RELIEF PLT WAS THANKFULLY ABLE TO TROUBLESHOOT THE PROB; REDUCING THE CABIN PRESSURE DIFFERENTIAL AS WE DSNDED TO A LOWER ALT OF 20000 FT AS WE HEADED BACK. HIS ASSISTANCE WAS CRUCIAL IN HELPING RID US OF A VERY SERIOUS PROB WHICH COULD HAVE HAD SEVERE CONSEQUENCES IF COUPLED WITH ANY OTHER ACFT SYS MALFUNCTION. HIS ASSISTANCE WAS INVALUABLE TO OUR CREW FOR THE TRIP BACK. WE WERE UNABLE TO CONTINUE N OVER SOUTH AMERICA AT THE LOWER ALT IN LIGHT OF THE WX THAT HAD SETTLED IN FOR A LARGE PART OF THE NORTHERN HALF OF THE CONTINENT. NOT ONLY DID WE HAVE TO ENDURE AN HR AND A HALF FLT BACK; BUT ALSO HAD TO HOLD FOR OVER 2 AND A HALF HRS IN ORDER TO BURN OFF FUEL. HAD OUR RELIEF PLT NOT BEEN ABLE TO PROVIDE HIS KEEN INSIGHT INTO THE PROB; AND RESOLVE IT IN THE MANNER HE DID; WE WOULD HAVE PROBABLY HAD TO DECLARE AN EMER AND LAND OVERWT SEEING HOW EXTRAORDINARY THE NOISE WAS COMING OUT THE CAPT'S WINDOW. I AM REQUESTING A RELY TO MY INQUIRY AS TO WHY THIS AIRPLANE CONTINUED TO REMAIN IN SVC WITH HAVING HAD 4 PREVIOUS COMPLAINTS OF EXACTLY THE SAME NATURE. THIS IS A VERY SERIOUS SAFETY OF FLT ISSUE AND REQUIRES AN INVESTIGATION AS TO WHY THIS WAS ALLOWED TO HAPPEN.