It may be due to DC in your powerlines. There's an active thread on this in Tech Talk. I had this problem with my Isoclean power conditioner. Inserting the Channel Island XDC-2 between the wall outlet and the Isoclean completely eliminated the problem. But the XDC-2 must be used within its specifications. Mine is used only for front-end equipment.
Ironically, transformer-based (balanced) power conditioners are subject to DC-associated transformer noise.
Ironically, transformer-based (balanced) power conditioners are subject to DC-associated transformer noise.