Sound quality issues.

I have an 06 LE with rockford system. I occasionally get poor sound quality from the front dash speakers, seems to be the drivers or center. I sounds like noise you get from a typical blown speaker, but is only on certian cds or mp3's. I is int and not always on the same disc. These a new cd's not burned copies so the disc is not the issue. Took the covers off and there is no debris or visual problem with the speakers. Maybe the sound amp is at fault? It is so int that the dealer cannot hear it, but it bothers me. Let me know if anyone has ever had this issue. Thanks.