squealing

OK so I have a question. I just got done hooking up my speakers. I have a Chevy cavalier and whenever I start my car, a squeal emits out of my 6x9 speakers but not my subwoofer, they are on separate amplifiers. The 6x9 speakers are on a 500 watt amp and my 12" sub is on a 200 watt amp. When I...