For what it's worth, I've had this same problem occur, and it happened for the first time after I upgraded to OS 7. I switched music apps and haven't had the problem since. I assumed it had something to do with the music app being outdated and not necessarily a Mackie issue. I was using an app called "Mp3 Player" when I had the white noise problem intermittently, but haven't had any issues since switching to the Louder Logic app. I started with the free version, but upgraded to the paid version. I've done several gigs without a problem, since.