You can try different power sources for the radio. Some of the 'hot' wires carry more noise than others.

You can try a BFC (Big Friggin Capacitor) from your hot lead to your ground. This should filter low frequency noise to ground. If it's an electrolytic capacitor, make sure you don't reverse the polarity.

You can try new spark plug wires - they are the usual source for noise bleeding through to your radio.

Most of the better retailers should be able to sell you a filer circuit if all else fails.