I listen to quite a bit of AM radio when I drive. AM reception in my area is not very good and on long drives I lose my normal 24hr news station. So I use the iphone as a digital radio. However, I get a bit of engine noise on my iphone when its plugged into the 12v socket. This is not uncommon, but annoying never the less. So in case you play your iphone or other mp3 player in the car while getting power from the cigarette socket; here are a few tips..
1) Turn your device up as loud as possible and manage volume via the car stereo.
2) If your car is older, you can reduce noise by replacing your spark plugs and distributor wires, to noise suppressor type wires. If you have these types, perhaps its time to replace them.