Mine does it on all speakers, regardless of the source.

I'm going to have to go back through my troubleshooting to see if I can narrow it down. I recently added an APC power center thinking it might be noise from the power line, but that made not difference. It's not a ground hum.

In fairness to Outlaw, I didn't even mention the hiss for the repair. That was a different issue - a loud screeching noise. I thought they might be related, but they apparently weren't.