Pretty sure SilverAthlon said it best here... If they are run on the same side this could be an easy fix...SilverAthlon said:EMI - Electro-Magnetic interference. Are your speaker wires insulated and cut/nick free? Is the audio signal cable from the head-unit insulated? Did you run the audio signal cable down the same path as the 12v power from the battery? This is one of the most common causes for this type of reaction. You might try isolating the cables from power as much as possible. I ran 12v/Amp remote feed down the left side of my vehicle, and the audio signal wires down the right. It's a lot of work but it's worth it to be able to rule out this kind of issue.