Corsa 2011 diesel Gearbox whine

[Fuzzhuzz]

Hi Guys,
My first post, hope you can help.
I bought a dealership demo Corsa DT 1.3 with 8000 on the clock. I have now done 22,000 and get a whine from the gearbox while driving in 5th and 6th gear. I took the car, still under 3 year warranty, to a different service agent for convenience , and was told after a test drive there was nothing wrong, the noise was usual.
Has anybody else experienced this annoying trait?
I should explain I am on an extended time away from home and can only return the car to where I bought it after the warranty has expired.
Hope you can help.
Regards
John