hey i have 2006 cobalt ss supercharged with the oem pioneer system. i have a aftermarket sub and amp along with a head unit. i am no stranger to wireing and stereo's.
but i get this noise in my speakers that comes and goes when ever it pleases. now i bought the car and it had an afftermarket head unit allready in it but it was a crapy one.
so i changed it and this is how it went. took old one out. found that who ever instaled the old one cut the harness to the factory head unit... (that makes for a fin time i gess) so i trace all the wires. the guy had 3 conections per each wire.... ok not a big deal so i re do them all to just one conection per wire like it should be. put my new head unit in and all was well for about a week then when i hit a bump some scratcy noise comes from the back driver side speaker. thought it was a loose conection. but kept driving it and it started to do it between all speakers. some times it will go away when i drive and otheres not till i shut the car off. and you can here the alternator through the speakers. but some times it will come through front left then stop and come through the rear right and so on. check all the conections at the head unit and at all the speakers. everything is fine and has a good conection. tryed moveing around wires but nothing happens. but when there is a big change in voltage it stops. ie. turn the heater from 0 to 4 it will stop but still comes back after a while. checked all my main power and ground conections too and can not find anything wrong with them. iam just wondering if anyone els as had or even herd of this problem? could it be the factory amp doing this?
thanks for all the help

 

if he had 3 connections per wire like you said he probably burned some channels out on the amp...