Most of the time you can reduce the noise a lot by using a LC filter in the power line. Go to almost any car audio place and buy one, or just use a old transformer and place the secondary winding in the line to the regulator, and a 2200uF 25V capacitor after it as the filter. Insulate the unused primary winding, and almost any transformer that provided between 6 and 15VAC will work. Brute force approach, but it almost always works. as well make sure you take a fused feed direct from the battery ( a 5A fuse in an automotive holder right there as well) or from a switched supply like the accessory lead for the radio, and a ground wire to the chassis of the car.