Hello Great Ones,
 
I began experiencing a strange "white noise" or "static" sound today when playing back about 10 trax. I isolated what I thought was the culprit track; i.e. the noise only came when I played that track.  Since I couldn't think of any way to get rid of the noise, I reinstalled the Quad Capture driver and initialized its settings.
 
Now, there is a strange overall echo sound when playing back audio.  Also, I cannot turn up my monitors past a certain point without experiencing a deep overload/feedback sound that could damage my speakers.
 
In the QuadCapture interface the ASIO settings seem to be correct and latency is good (none).  I have the "Use ASIO direct monitor" checked.  Sample rate is 441 and the clock is internal. "Low latency" mode is not checked.  I have changed nothing else in my system and this is the first time I've had this problem.
 
Any suggestions, please?   Thank you

but if you mute that one bad track, all the noise goes away?

could be something trivial like a mic still plugged in some where (with input echo still on) and nothing to do with your previous problem (the static).
 

Shot in the dark, but since it's a bus-powered unit, might be worth switching USB ports and see if that changes anything.

As said above, it could be your monitoring system, actually if there is feedback this more or less proves this theory. De select the direct monitoring. 
 

We are recording tapes with 3 Quad Captures and there is a little bit "white noise" and a static buzz 200 Hz (?) in the monitor out.
But when I load this recordings on an other computer this recordings are all absolutely clean without any noise or buzz. So I think this is a problem with the monitor from the Quad Capture.

Just in case the white noise is due to a trial plugin ?

Because the problem persists even when I playback from another DAW, the "trial plugin" idea doesn't seem accurate.
 
Thank you all for your responses.  I suppose it could be due to a bad Quad Capture unit.  I hope not, though.  I am pursuing this further with Cakewalk support and I'll let you know how it goes.  
 
Thanks again! 

What happens when you move it around. My quads monitor out as picking up something from a secondary laptop. Moving helped me find it

The QuadCapture is microphonic and hums very slightly in Hi-Z mode.
Turn off the Hi-Z switch at the back of the unit.