Loud noise when new head unit is powered up and track changed
 

I have just installed a new Alpine head unit for my audi a3 1.8 t sport 1999 model and when i power the unit up, or change track a short, butloud noise comes from the speakers. The standars is a bose fully amplified system. I think it may be something to do with the standard amplifier.
can anyone tell me why this is and help me fix this?
Thanks