well Id start by pulling the head unit and looking at how its hooked up and look for loose connections, if you have another head unit try wireing that up if the problem persists you might have blown speakers. does it have a poping sound with the car running or off? I know alternator whine is a common problem if you have a bad ground, you could also reground the head unit yourself if you wanted but Im betting that something is wrong with the cd player on the h/u its self..