I got a v2 STI and when i pull the clutch up quickly or accelerate or decelerate suddenly, i hear a banging sort of noise. i have checked mountings which are fine. ive had my dad underneath while i got it to make the noise and he says its in the gearbox area

 

doesnt sound good mate gearboxs ar common to go on versons 1-4

 

Mine does the same but only after I had my clutch fitted. Be interested to know whats causing your problem

 

I had the same problem with one of mine a few years a go and it turned out to be the rear diff beam bushes I would double check all mountings first.