I have a 2.0l glt and have a similar noise, I am fairly sure it is a bearing or two either side of the diff. (bottom rear of gearbox). I asked a mechanic I trust and he told me to leave it, dont waste your money, if it gets worse it will be easier to find whats wrong, I had jacked up each wheel in turn and run with one wheel off the ground and noticed no difference so either I had two identical bad wheel bearings (unlikely) or a gearbox noise and is likely to be near the diff as it relates only to roadspeed. If its not too bad just make sure you have oil in your gearbox and either cross your fingers and leave it or spend money on a repair or good second hand gearbox If you find its the same problem please tell me how you get on