If it is the box, it probably going to be better to take it somewhere - just only of you are sure it is the box. It could also be the drive shafts and these tend to be a lot easier to replace than gearboxes.
Simple test. Jack up the front end of the car so that both front wheels are in the air. With the car in gear (engine stopped) rotate one of the front wheels. The other wheel should rotate in the opposite direction. If there is any noise - it is the differential in the gearbox or the drive shafts. Most of the gearbox will be stationary.
Take the car out of gear. Have a mate hold one of the front wheels to stop it from moving. Rotate the other wheel. If you hear the noise and you didn't previously, then it is the gearbox. If you heard then noise before, and you still hear it, then it is likely to be a problem with thet driveshaft of the wheel you are rotating, or the differential. Repeat, rotating the previously still wheel and holding the rotating wheel.
You should be able to norrow it down to either drive shafts and/or gearbox. With a bit of extra effort using a mechanics stethoscope(sp?) you should be able to narrow down the source of the noise further.
Good luck.