I had this issue with a fairly recently manufactured Hermod in an Intellijel 7u case while using my OP-1 to input MIDI via USB. It drove me crazy.
I dug through the olā bin-o-cables and found a USB cable with a pretty large ferrite bead right at the USB-A end and it completely eliminated the noise (audibly anyway, never recorded the system at this point or checked outputs on a scope).
I remember chuckling because it seemed like such an obvious and crude solution. It didnāt save the Hermod from being sold, but I was relieved to find it wasnāt specific to my Hermod or OP-1.