Hmmm, what your describing sounds like a broken or chipped tooth.
Get it confirmed as that, your welcome to bring it in to us at Subaru Wangara. Wont cost you anything for me to go for a drive with you or listen to the noise.
If indeed thats the problem, it may be cheaper to get a second hand box.
First gear damage requires replacement of the main shaft.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i