My 1.6D Bravo gearbox started to fail at 100K miles. The sound it made was basically a bearing but sounded like music. I did quieten this with a Moly* gearbox additive. I sold the car nut I think it was scrapped a year or so later. If its just a gearbox bearing is is fixable. I should have had the gearbox overhauled as it was a really great car. I f not the gearbox bearings then I would be looking as Busymike suggests. My car made no other noises in the other gears. I jacked up one wheel and it could be heard when the wheels were turning in those gears engine ticking over.