i dont really know what you mean by "exhuast noise" (cos..um...they should make noise?) but what i think you mean is a much more pronounced noise...
anyway, how much do you know about tuned resonance? basically, the 'volume' of the exhaust obviously changes as the rpm goes up. manufacturers have to abide by both noise and emmisions regulations, which are tested at certain rpms in different countries, the manufacturer tunes their vehicale as such which can lead to "flat spots"(if they know they will be tested at 3000rpm eg, they will lean out etc to make sure it within emissions at that point) and also sometimes the resonance of the exhaust will change dramatically at certain rpm to change the frequency of the exhaust note to get past noise regulations. your "exhaust noise" is prolly due to the fast that the astra is in reality and VERY quiet car for a turbo, and has possibly tuned the exhuast note to be slightly louder at 3000-ish rpm compared to higher/lower.
but thats just me putting a logical spin on things...my exhuast is custom made, and is very loud when driving..but is somehow almost dead quiet between 2000-3000 when cruising....