from my experience all active powered speakers have a noise floor which means you can hear it with the distance depending on your hearing. In the past ai have been able to diminish it a bit through a combination of using a good source component (DAC or amp) and power cord and cables. However, you can minimize the noise not eliminate it. This assumes the amps on your speaker isn’t damaged...

Edited.

Thanks for respons, friend of mine has beolab 9 so i could try setting them up at him and compare if it is any difference on the noise. 

I would surely check my power wiring layout to eliminate potential ground loops before spending on fixes. My cottage was renovated twice so I have dealt with the mysteries of what goes on behind the walls. In my case it cost me for 2 longer power cables and a long-cabled surge protector to get all my kit on one circuit, but the hum has been gone for a long time. Not bad.  

You can get hum when electronics are on different power circuits, have dealt with that in the past. Ground is not always at the same level on different circuits. But I think the OP is talking about hiss, the kind of random white noise resulting from random thermionic noise in most electronic circuits. I can't hear it in my 9s or 8000s, but then I'm also old and don't have the HF hearing ability I had in my youth, back when ultrasonic burglar alarm sensors felt like ice picks being shoved in my ears. One of the few advantages of being advanced enough in age to be of the pipe and slippers crowd. 😠

The way you describe the sound michael is pretty much spot on. My friend who has bl9 got the same noise, but you have to put your ear close to the speakers too hear it. I have no problem hearing it 10 feet away. But its like you say, faint low river water flow kind of noise.

I dont think its ground related. Not really sure how to explain it, but im sure you know what i mean. I borrowed a transformator that split the power outlet from the ground net, but still make the outlet connect to ground. The bl20 doesnt have a powersocket that connects to ground so i think it even doesnt mather. Not really sure though.