Plasma tv & preamp hum-not a loop

Can tubes pick up some type of radiation from a plasma tv? I had to move my entire system to refinish the floors and when I put everything back (all exactly the same except perhaps the same plugs in the power condition were not used as before the move)I sometimes get a hum from both speakers. Mute the pre and it stops. It comes and goes and varies in loudness. Sometimes it just stops only to start again a few minutes later. I know it is not a ground loop because of its behavior and sound. It does not seem to occur as much if I warm up the pre for 20-30 minutes. Thanks for any answers.