Can't anyone help I'm having a like a whirring noise when accelerating and letting of the accelerater but the car is driving fine and pulling well not sure on what it is.

Hi all - just recently got Astra SRi CDTi 120 and am looking for advice as to what upgrades/improvements you would recommend. I know the car will need a back box fitting soon, so wasn't sure if to get a sports back box, or full exhaust system - is it worth it? What costs/improvements would I...