A few questions:

Is this noise a new problem? Did your helicopter use to sound normal? How many flights on the machine before the noise appeared? Did the noise appear after a crash, incident, rough landing? As to the gear mesh other have mentioned, this part of the setup is critical. You should just barely be able to feel any play between the pinion and main gear - paper width as others have mentioned. If your machine has hundreds of flight, you need to also be suspicious of all ball bearings and even the motor. Best of luck!