I recently bought a HPM Entertainment Powerboard because it came with a RFI Power Filter.
RFI stands for Radio Frequency Interference.
On the back of the packet they give an explanation of RFI
” Radio Frequency Interfernce ( RFI or Electromagnetic ,EFI) can be caused by radio signals , mobile phones , wireless routers , some weather conditions , motors in household appliances , fluorescent lights , loose wiring conditions , power surges and spikes.
In Australia electricity is supplied to your power points at a frequency of 50mhz.
This signal can become noisy due to RFI , meaning the signal is no longer a clean constant 50mhz.
This power board has a torroidal choke coil to help provide a clean power signal to your devices.”
If you experience a hum or feedback on some of your devices this maybe the answer.