crackling sound in front speakers

Hey guys. Been a while since i posted here.

My friend was having a problem with a crackling sound coming from the front speakers. mostly front right. But not on all songs. just some songs. So i took a look, changed some settings. so i figured the speakers were blown. So i changed them out with new ones, and it worked fine. this was just a few days ago. She just called telling me the same problem was happening.  Anyone have any ideas? Wiring touching? cd player? I'd like to get some help before i rip her system out only to find it's something small and stupid.

Hehe.  Problems like this are always something small and stupid.