Bandit, have you tried charging your Dinc in the car & listening to music using the car's speakers? (I have an audio input jack in my car, but I hear a humming noise when I hit the gas pedal).
Also, do I really need a case/skin for the Dinc? I mean, I'm very careful with my devices. What does the back off the phone feel like? Is the case slippery? Or does it grip well?
The winding sound you are hearing when hitting your gas pedal is caused by a unshielded cable somewhere. It could be your RCA cables if you have any of your cable going to your phone. It isn't the phone I don't think.
As far as if you need a case it's all up to user opinion. Personally I always try to put at least a slim case like an Incipio Feather or a Seidio Inno II on my phone for just enough protection. You don't need like a Otterbox level protection unless you are careless with your phones, clumsy, or over protective. SO it's really up to you if you need one of not.