I have a Behringer UCA202 (no-external power) USB Interface (DAC):

Bus 006 Device 003: ID 08bb:2902 Texas Instruments Japan PCM2902 Audio Codec

and an Intel audio controller which is built into the laptop.

00:1b.0 Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller (rev 03)

When I plug the both of them into the mixer, I get loopback. It sounds like a high pitched oscillating noise. I'm wonder if anyone has any idea of what this nose could be from and how I further trouble shoot it. A simple configuration with the mixer to Behringer and audio-out on laptop to Mixer in can cause this feed back.

Is this nose perhaps because the line-out on the laptop and behringer In have two different fake grounds?

Any ideas are appreciated.r