[mdoyle]

I just re-read my reply and noticed I said "alternators" when I should have said "distributors" in the Corvette shielding example.

Spark plug wiring is usually shielded, noise suppressive but if you've got a cracked wire, coil, or bad plug then you'll notice an increase in noise on audio equipment.

In some cases you can pick it up outside the vehicle on the AM spectrum using a radio. If you have a long wire connecting the device to an audio input, that will sometimes act as an antenna.

One quick and dirty way to check for damaged ignition components is to observe the motor running in low light or darkness. Look around the spark plug boots and ignition coil(s) for any arcing. If you see a spark jumping then you've likely found the source of your interference.