Arming tracks produces noise

I've been watching the Lynda tutorials and really digging into STP. However, whenever I arm a track in preparation for recording, I detect a very high pitch, background whine which will not go away.
I'm monitoring through an Alesis usb mixer which I use for my record mics. All my preferences have USB recording and monitoring enabled. I'd like to stick with STP but the app is introducing noise that cannot be considered professional audio quality. What could I be missing here?

Thanks!!
Mark

At one time the usb alesis had protocol problems in 10.4. I don't know about now(10.5).

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