When I purchased my z4 it already had an iPod kit wired in, a 30 pin Apple connector that played and charged iPods and iPhones. Previous owner told me it was a Denison product. No problem with it over the last 18 months but recently I've noticed there's a slight humming noise that disappears as the revs increase. Soon as I'ms idling again, it's back. This doesn't happen on the radio input either. First thoughts are some sort of grounding issue? Is there a guide on accessing the CD changer from the boot?