MacBook Crackling Speaker on *some* songs ...

Hey,

My Macbook ( white body ) is 6 months old and there is a crackle noise coming from the inbuilt speakers on the some songs ... some songs which are much more "busy" don't have any issues and sometimes I can play classcal piano with a crackle but loud rock music is perfect..

I'm strugging to understand if this is a hardware issue or whether it's due to the sound quality of some of my files - I have a really mixed bag of itunes from various sources ..

Does anyone have any experience with this type of thing?

Thanks,

Matthew