I had this problem on my Oculus Rift box (remember those? it still runs beat saber just fine) and the solution was to solder some beefy capacitors on the end of unused power cables coming out of the power supply. If I recall correctly it was the 12V line that did it, which I didn't expect.

The buzz isn't completely gone but now I can't hear it unless I'm paying attention to it, which if I am playing beat saber, I'm not.