aux sound problems

Hi I have a Laguna iii with a aux box in the glovebox when you plug say a 3.5mm plug in then your iPod the sound quality is so bad then if you drive of it comes fine to it is left after switch of of the car then it starts again it is like distortion for 5 minits is this a problem with the box because I have changed the 3.5mm cable so many times now any help thanks