What is Causing a Noise Signal in USB Audio Recording?
It's NOT USB problem. If you there was anything wrong with the digital connection, you would get drop outs or nothing at all. The error detection/correction on the USB bus is binary: either you got all the bits or you didn't.

This is clearly analog noise, probably from switching power supply. It happens most likely in the analog front end of the mixer. Once it's digitized, it can't pick up this type of noise anymore. 

You can try a different USB recording device or throw a USB hub in the path to isolate the iMAC power from the Xenyx power see if it helps. It's also entirely possible that the Xenyx will do this no matter what: Behringer has a reputation for great value, but also for sloppy design and poor quality control. The Xenyx is a blatant ripoff of the Mackie Onyx (as the name implies).