Hint, Single Coil pickups will pick up electronic noise like a magnet! When at a right angle to electronic source, it will not pick it up as bad, but it's still there.
Now synkrotron have you tried to unplug the cables from your audio interface to your speakers and listened to see if the sound is still there. If it is, then it's the speakers themselves. Although I am doubting this to be the case (I mean I can't see both speakers generating the noise, but you never know). It's worth a check.
If you try the above and don't get the sound, hook the cables back up to audio interface and unplug everything else from the audio interface. Still hear it? Then I would look at the audio interface. If it is the audio interface, I would suggeswt getting a ground lift plug adapter to try and see if using it will eliminate the noise.
The things I am suggesting are efforts to the find noise source. Once you have the source, you can figure out the solution.