Maybe God doesn't like you...did you run over a squirell or something? j/k There's a pretty easy fix to this problem...mine has it too...
1) don't drive it immediately after starting it, EVER--let it warm up a minute or two
2) if its cold, push the clutch in (the sound should go away) and wait a few minutes until the engine warms up..the tranny won't exactly be warm, but it'll be warmer..then just drive very gently for a minute and it should go away...

I HATE doing this, but i'm afraid i'm gonna chalk up the ratllin' tranny to one of those "noises" you just have to turn your stereo up to and ignore

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Black 98 GST missing her owner and looking for a new one...Inquiries welcome