My car makes the same noise - its the front gear box bearing. If you were to put your foot on the clutch lightly and it made the noise then it would be the release bearing.
How do I know? My dad is an automotive teacher at TAFE - comes in handy let me tell you!
No need to replace until it becomes excessively loud - mine has been making it for around 15,000km now and is getting no worse - can only hear it in the garage or really late at night going through a tunnel or something. Dad advises that once you drop the gearbox it is easily accessed and the bearing is cheap!
Hope this helps!