I did wonder what that mosquito noise was
Fixing that wasn't "free."
MultiNotch written by Steve brutally smashes certain musical tones that we determine were the worst offenders. They're just gone. We're hoping nobody looks too hard at the show and you don't try to send music through the system.
The tones are 1000Hz, 2000Hz, 3000Hz and 4000Hz. Those roughly correspond to the signals that USB uses to do its job. You might know what 1000Hz is. That's one of the tones Broadcasters use, for example, behind colorbars as a test signal.
The more normal test tone is 400Hz. Never being one to follow the pack, NBC New York used 400 and NBC Washington used 440Hz.
You totally know what 3000Hz is. That's a baby screaming on a jet. That's one of the reasons this whine sound can be low level and still so irritating.
Normal piano tones don't ever land exactly one one of those tones, but if you have an instrument with no fixed pitches such as a violin or a slide trombone, you could hit musical tones that don't make it into your show.
And you thought this was easy....