I never once agreed with assumption about ground loop. OP made it clear that macbook runs on battery power with only USB audio alone. It's unlikely to be ground loop problem./p>
The OP's info was several posts in coming, in fact, still incomplete.
If anything, macbook or usb audio device may have problems with USB audio implementations.
Possibly, but the typical problem you have with a Mac and a USB device results in no audio at all, or horribly distorted/noise audio from a mismatched sample frequency.
Also, shielding isn't only for EMI/RFI protection. Connecting ground chassis between device can reduce interference inside unit with proper designs and some documents recommended to connect shielding to both sides. Let's see if using shielded usb cable will solve it. It's only $5.
You have an incorrect concept of what a shield is for and how it works. The shield is an electromagnetic/electrostatic shield blocking electromagnetic radiation with an electrically grounded surface, and blocking capacitive coupling by grounding the electrostatic field. Pin 4 of the USB connector carries ground, shield or not. Ground is essential for USB to work. The shield is not required for the ground connection to work. However, the shield must be grounded to function as a shield. The fact that it's grounded at both ends of the USB cable creates a problem: a potential ground loop. In audio, we often lift the ground connection of the shield at one end of the cable to prevent ground current from flowing in the shield.
However, sometimes...
sometimes...the shield may provide a lower resistance ground path between two devices that do not share an otherwise common ground. While you may
think you've "solved" the problem with a shielded cable, what you've actually done is provided current path from one device to another that runs not only right next to the signal wires, but has no shield between it (being the shield itself) and the wires, thus having both capacitive and electromagnetic coupling to the signal conductors. This may or may not cause an issue, but the potential is there for slightly corrupting the transmitted data, possibly creating a jitter-like anomaly.
The correct solution is to get both devices to the same ground potential without using the shield to do it. In most cases, it doesn't matter, because the ground current is low, coupling is low, and data isn't corrupted. But it's not the correct way to "ground" two devices together.
I would suggest we continue to collect information from the OP before slapping a $5 "fix" on it.
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