If its the turbo the pitch of the whine will vary with the throttle opening, you should be able to play a tune on it by varying the accelerator pressure, if its only there in 5th and constant, only varying with speed, it will be the gearbox, probably a noisey bearing, curing it would involve removing and dismantling the gearbox - or replacing it.

I know that you can get to 5th gear on the CX box because I have done it but don't know about the 406, its related so possibly you can, but, the problem won't be with the 5th gear itself it will be a bearing and as 5th is in an extention on the back of the box the bearings are behind it. It could be the diff bearings, the car is quieter in 5th so you may not be able to hear it in the other gears. If its not to noisy I would put up with it, gearboxes do sometimes get noisy with age and will still probably outlast the car.

Peter