Gearbox Whine

[Larry May]

I have recently encountered a strange whine from the car when accelerating from about 40-60mph, only in 5th gear. The rest of the gears seem fine just when the gear lever is in 5th it is a really loud whine gradually tailing off at about 60mph.
There is no change in the apparent performance of the car, pulling in each gear, mpg etc just this really loud whine.
Is this something I should be worried about, could it be a major problem.
I have recently had the water pump replaced after it failed could this be linked to it at all?