The buzzing has to be coming from relative motion or tracking/arcing. It could be some surface moisture causing some surface currents or depending on cage construction a relative motion could be occuring due to the rotating magnetic field developed when AC flows through a conductor and this field is pushing or pulling on some metal object close to the flexwatt. To stop the relative motion issue in my rack designs I use a spray adhesive to glue it down then I take it down with aluminum tape. (Make sure you cover cut ends of flexwatt with electrical tape first.)