Dean's idea is probably what will cure the noise from the alternator, I have used those before.
One idea that I have had some success with also may be free if you have some computer junk about. Many keyboards, power supplies, and other peripherials have a ferrite bead molded onto their cord. It's a tubular section about 1 1/2" long and 3/4" or so in diameter, usually right near the plug, or internal on many keyboards. Remove it from the cable, and pass the positive power lead through it, relooping if possible, as many turns as you can squeeze through.
It may not be as effective as a proper choke and cap filter, since it is designed more for higher frequencies, but if you have one laying around, it's worth a try and it is free.