Apple Logic Pro /LogicExpress Discussion

Hi 
I have used Cubase for years and I'm very new to this MacBook Pro, Garage Band-Logic 
experience so please bear with me. 

Why do I get static in my Garage Band Compositions? All effects are turned off, and it doesn't 
happen in every song. Sometimes the static goes away when I mute enough tracks; 
sometimes I must only play 3 tracks to get the static to stop. 

Any ideas?

Thanks!