How can I stop the humming on my ax m138 ?

When powered on there is a loud humming noise coming from the speaker mainly the sub. This goes on for several minutes and quieten down slightly and starts again and this circle continues. The fan don't seem to work anymore.. I don't hear it come on as it used to.
Can anyone please help as the noise is not very nice especially while listening at low volume.

Gikes
May 2018