I have the Mach 460 system in my '94 GT. This doesn't always happen, but several times per week when I start my car and the radio/cd player turns on there will be a background noise. It's not always noticeable at first. The weird thing is that if it's making the noise and I'm accelerating the noise will get higher pitched. If I'm decelerating it will get lower pitched. Anyone have any ideas what could be causing it? It makes the noise out of both lower door speakers. I'm not sure if it makes the noise out of the back speakers. Thanks in advance for your help.