Hello Claudiu,
Sorry for my English. It's not my native language.
I’ll try to describe this to you.

In my case the major problem was with the AC Socket.
You could move it. And by moving it the sound got worse/terrible.
It has been fixed by Roland.
I sometimes still have a little noise. The cables I use with my amplifier are connected with the plugs for the output 1 sockets. The connection between them gets corroded a little. This is the cause for that.

I hope I have been of help to you.

Gerard van Roemburg

By the way, did you try the advices i got from the other members?