The source is electrical noise, but the solution of isolating the audio chain from the computer's USB means that in the future you might not notice when you've introduced another GPU memory bandwidth hog into your rendering loop.
Good story, though.
That might be the strongest spacebar heating workflow situation I've actually run into so far
Just attach a scope to your power lines, boom, live feedback on what your renderer is doing.
I wonder if that will be next fad in PC building, just putting live power line graph on the screen inside