As above, very common, The reverse idler normally makes the noise, when it starts loosing teeth 
Does it pop out of reverse gear??
It should last you yet, if its just started to make the noise now, Soon or later it will need replacing with a new gear box or second hand unit.
As far as i have heard each time it pops out of reverse, you loosing a tooth, so eventually the reverse idelr will wear off and you will have no reverse gear.