Aux in - I POD Touch problem

[ghk]

I have a 60 plate A5 and am trying to connect my brand new I Pod Touch to the Audi sound system via the 'Aux in' socket. I purchased a connecting cable (Jack plug at both ends) from Currys, the assistant assuring me it would do the job. The problem is when i play the music the audio emanates fom the speakers but is distorted by a 'hissing' noise, although when I partially remove the cable from the Aux in socket the 'hissing' stops but the volume is very low. I have changed the cable but same result. Any ideas or is it back t the dealer?