Might be a differen AGE of manufacture causing the difference as the noise regs are tighter now, ie yours might be an older system.
I have just fitted a scorpion to my astra(enzo killer) and that was VERY quiet, upon enquiring at scorpion this is what they told me, as I mentioned the sapphy system was so much louder.
I realise they are totally different cars, but it seems EEC noise regs are to blame.
tabetha