I have an Focusrite Scarlett 4i4 external audio interface, which (important!) draws its power from its USB C connection to my computer's USB hub. I noticed that whenever I connected my piano keyboard's MIDI interface to my computer (also via USB), then the Focusrite added a 2400 Hz hissing sound to any signal routed into its Audio In. The reason was that this MIDI interface disturbed the ground signal in the USB system, which the audio interface needs to properly interpret the Audio In signal. Changing the MIDI interface (MIDI to USB) didn't change the problem, as the ground was disturbed already in the piano.This iFi Defender, removed the hissing sound completely. Two important things to consider: (1) I plugged the Defender between USB hub and the piano's MIDI interface (!), not between USB hub and Focusrite audio interface. (2) Make sure (!!!) you add a 5V (!!!) USB-C signal into the Defender's side; without it, the hissing doesn't go away; higher voltages short-circuited my system, causing me a major scare.So take a regular small (!) cheap iPhone charging block for this 5V side signal. I first used just some free USB power slot from my high-end Anker charger, which usually provides only as much power as needed, but more than 5V in this case apparently; here this literally overpowered the Defender and caused a crash; so don't make the same mistake.All that said, I am quite happy to finally having gotten rid of the hissing sound. The Defender seems expensive, but I am really happy I found a way to solve my problem finally! :-)