Isolate the noise. Is it coming from any signal on any line from your board? Use headphones and plug direct into your sound sources. Do you hear it there? Or somewhere else in the signal path.
If the noise is there BEFORE the board then what machine?
Make sure you are not running ANY audio cables over or under any power cords or power adaptors.
If its a ground loop from the mixer then use a 3 to 2 adaptor on the plug. You should also buy a power conditioner to protect your gear. You can get a furman with a couple of filters for under $50.
Do these conditioners filter the ground wire also?
I only have one grounded outlet in my flat which is used also by all the kitchen equipment and PC, so it's really dirty. Any other solution?
That's why at the moment my monitors are ungrounded, which is far from optimal
Has anyone tried anything like this:
1. Connect monitors and other audio equipment to a grounded power extension cord.
2. Connect the extension cord to an ungrounded power source.
3. Connect the grounds of the extension cord to an external ground (building structures).