@drewbert I heard you talking about hum, USB cable and MacBooks on the podcast.
The reason detaching that cable fixed your problem is you have a ground loop. That means that in your system you have multiple ground points which aren't at the same voltage.
You mentioned detaching the USB cable fixed the problem but wondered why that is when the cable had a choke/ferrite bead/inductor/filter on it. Those are used to block AC signals, that is signals with a frequency, typically a high frequency. When you hear beeps, chirps, etc. due to electronic noise, those are high frequencies or at least relatively high frequencies causing those. A ferrite bead used properly will fix those.
But you have a DC problem. The grounds are at different DC voltages. A bead cannot block those. You can block those using capacitive coupling, but that can be problematic to add on. Another way (that Jeff aluded to) is the to use a transformer, like an "audio matching" transformer or similar. Someone who did car stereos might know about that and Jeff seemed to reference that. The problem with using a transformer in this case is that transformers have a cutoff frequency. As long as you are passing audio only over the transformer you can make this work, but with USB you are passing very high frequency signals and it really won't work.
So what you need to do is to eliminate the ground loop. There are two ways to do this. With properly made audio equipment the right way to do it is to bond all the grounds in your system together with a nice thick ground wire, like 14 or 12 gauge. Then all the grounds will be equalized to the same voltage and the loop is (probably) gone. Significant current might flow through the bond, that's why you use thick wire. But the problem with this is that if there is intentional current flow over a connection then the grounds will still differ for reasons I won't get into. And with USB the device is probably drawing power, so that means intentional current flow.
So you use the other fix. That is lifting all the grounds but one. That means that every ground but one becomes a non-ground and all the devices then ground through that one connection. This is only okay if there is not significant power draw across the devices, because this means that devices will be using each other as a path to ground. In your case, this should be fine.
Now, let's talk about how to do it and MacBooks in general. In a MacBook, the audio ground (more accurately called an audio return or reference) is capacitively coupled, so you don't have to worry about it. EXCEPT that if you have an audio device that you connect that has a metal face on the plug (on the part of the plug you grab) then that metal face can touch the laptop case and that case is directly grounded, not capacitively coupled. To fix that, either pull the plug out very slightly (half a mm will do it) or paint the face of the plug with nail polish to form an insulator. Or put one of those paper hole punch reinforcers on the laptop or plug face instead of nail polish.
Next step is to to lift the power ground to the wall. The easiest way to do this is to just unplug the MacBook. It has a battery in it so it'll still work. The problem is now the MacBook will run down. So to fix that, stop using the 3-prong cord on your laptop power supply. Switch from that to the 2-prong mini-blades you got wth the power supply. This doesn't have a ground, so it lifts the laptop ground.
If you use the 2-prong power blades and make sure the audio cable (and anything else conductive) isn't touching the MacBook case, then your hum should go away even with the USB cord attached. Now you just have to check to make sure the laptop case isn't live (with a meter or simply by touching it) and you should be good to go.
For more info or how to find out about some more details behind this problem and similar ones, you can message me.
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