Ferrite!

I've got ferrites on a lot of cables, especially those connected to the Raspberry Pi.  But today, I found a spare ferrite and decided to clip this on to the rig end of the USB connection to the soundcard; I already have one at the Pi end.

 

I was quite amazed to see how effective this was at 28MHz. The ferrite is a TDK ZCAT 2032-0930, 9mm inner diameter.  With single board computers, which don't have room for good RFI control, liberal use of these ferrites is much advised.