I have an iPhone 4 and use a Magellan GPS dock for a speakerphone, charger and to output audio to my car's sound system using a cable with 3.5mm jacks. However, this was true before when I just used any cable to plug my phone up and charge it during driving. When charging the phone, I get this annoying high-pitched hum through the sound system that goes along with my engine noise. So if my car down-shifts, you can hear the hum change with it. It's not so bad on a lot of stuff, but on things like classical music that require a higher volume, it can be unbearable. I wanted to know if anybody knows a good way to get rid of this interference. I know you can get rid of GSM buzz on computer speakers by using iron ferrite beads on the speaker cables, but I don't know whether that would help in my car. It's directly related to the phone getting charged, so I'm at a loss as to how to do this without unplugging it. The GPS cradle won't do anything if it doesn't get power to it, so that's not an option.