I didn't. I would like to install some low-pass filters on each wire to speaker, but I don't know what exact filter it should be.
@videojo I believe that you did some amplifier upgrade. Did you have a similar audio issue?
I have attached a recording of this strange sound - increase your volume level to 100% to hear it. At the end of the recording there is quite break when this sound disappears before next song starts.
Yap, it this the exact same sound my unit makes too. It almost sounds like coil whine.