08 LT standard radio...I installed an Alpine power pack ( the one that hooks up right behind the HU) and all was fine for a month, but now an intermittent buzzing has been driving me nuts. I think it has something to do with the on-star, but I just let that lapse. Bad amp?? Any ideas??

 

Double check the connections, especially look for a poor ground. If the noise increases with the rpm of the engine, you might need an additional filter/capacitor on the alternator.

 

All that checks out...also it continues to buzz when I turn the key off and remove it...stops when I open the door