When I play my iPod the sound is terrible. Vibration etc. on 2007 BMW 335xi

The radio and cd's sound fine. Dealer blamed the cable connecter but upon swapping with a friend it made no difference. iTunes or downloaded cd's are all that's on the iPod so good sources. Help!

Asked by for the 2007 BMW 335xi
Couple questions:
Is this a factory ipod connection kit?
Have you tried a different ipod?

Yes it's a factory connection, and the iPod is new- 6 months. But no, I have not tried a different one.
I'd try a friend's just to rule out the connector on the bottom of your ipod before chasing other potential causes.