@Marek.Danko
I guess ADI-2 DAC FS has similar powering like ADI-2 Pro. Or other RME units with external power source.
As far as I understand it those RME units have internal power unit that makes the powering, unit needs, in quality it needs. ADI-2 Pro may be powered by wide range of voltages and those may be the both DC and AC. So, it is clear that quality of external power source is not esential.
Maybe that better external source makes things better, but I sort of doubt it....
But on the other side, you are right that power source should not "whistle". It indicates, that not everything is 100% right. If it is under guarantie I would ask for replacement. In other cases, I would just switch off the mains.
If you are affraid, replace power source by new one with similar spec. Or by the super power source some people prefer to use with audio equipment....