06 Plate GTI with Whirring noise

[Gilbert]

Help, I have an 06 GTI that has developed a whiring noise at low to medium speeds. It sounded like Wheel bearing noise but I have replace worn bearings and the sound is actually getting worse.
My mechanic "thinks" it may be gearbox and the noise reduces when i get into 4th which also means i am travelling faster.
Any ideas ?
Thanks