I have a set of KRK Rokit 5 speakers (they are self powered studio monitors, probably about 5 years old.
They have a fair bit of irregular buzzing/static noise. Not extremeley loud, but enough that when I sit near them it is quite distracting.
I noticed that it happens whether or not the speakers are connected to an audio input (which would be via an RCA cable right into a headphone jack). And it also is independent of the volume adjustment on the back of the speaker. I have tried a few different power cords, including a surge protector.
I'm not sure what makes sense here, other than maybe it is an issue with the speakers themselves. Both in the pair do it, but one is worse...