TD5 Gearbox Whirring Noise In 5th Gear

Did a search and came across an old 2010 thread which blamed bearings and suggested replacing the gearbox but not sure if it applies to my problem.

I've got a 2001 TD5 with 98K and a full service history. I've had it a couple of years and it has always had a loud whirring noise when 5th gear is selected and the throttle is pushed down. However as soon as I take my foot off the throttle the noise stops and immediately reappears when the throttle is even just lightly touched. All other gears are fine with no noise.

Is it a mechanical/gearbox/bearing problem?
Why does the whirring noise cease when not on the throttle?
Am a mechanical numpty so please be gentle.

Thanks in advance.

it would sound like box needs rear extension housing off at least ,to look at 5th gear and bearings ,you get a difference between power and overrun on the loading on components,

Thanks Jamesmartin. Will speak to my local garage and may get it looked at at its next service.

Hi, did you ever get to the bottom of this issue?
Intrigued as I have similar issue with 5th and 5th alone. Whining/whirring noise under load.

Cheers Lyndon