Gear box started making some clattering noises

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Hi there,
I need a little advice on what to do about some clattering/whining noises from the gear box.When I engage 2nd gear I suddenly get a slight whining noise and in 3rd it sounds as if the gear box is grinding corn Shok .I'm assuming it to be the gear box as I checked everything else up to the prop shafts to see where exactly the noise is coming from ( as I couldn't even locate it properly even by standing under the running vehicle on a lifter).Is there any way of a 'Quick-fix' or is it best just to replace the whole unit?And before I forget to mention it the car was first used by a trailer hire firm but has only 159.000 done so far.My first truck went out with 389.000 on the clock...Any help or advice would be appreciated.Thanks

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