Fifth gear whining

Any help would be appreciated.
when changing gears all seems okay as new clutch flywheel done at 101,000 miles and car has done 125,000 miles only on fifth gear when changing and driving and realizing accelerator is whining. I have tried to look on the internet for a research but nothing. Mechanics think it is gearbox synchronizer some think gearbox cable. I have even changed driveshafts both front. No idea what to do. Please let me know. Many thanks.