GM factory stereo buzzing

I have a 2005 Chevy Equinox with Onstar and factory subwoofer. There is a buzzing noise coming from all speakers, but it is the worst coming from the front passenger tweeter. The buzzing starts as soon as the key is turned to the ignition position (before and after the engine is running), but not in the accessory position. RPMs do not change the frequency of the buzzing and I have tightened all the grounding locations. I have also made sure that OnStar volume is turned all the way down. Any help or suggestion would be greatly appreciated. Thanks!

If it's the factory stuff, most of the speakers are probably dry rotted and/or blown.