I had this same exact problem when I connected my Ipod/Iphone via the USB.
The problem was with the cable I used. I went inside and used another cable...a white apple USB cable and the popping/scratching went away. I was using the cable that came with my aftermarket charger and for some reason it was giving me the cracking and popping sounds. It really sounded bad....swtich your USB cable to the original apple cable..and see how it sounds...
