Hum in ARC Ref 3 not on 0 but 1-103 light buzz

I'm suddenly experiencing a strange phenomena with my ARC Ref 3 (250 hrs on the tubes). On the 0 setting of the volume "dead quiet" with 1-103 a constant sound like the sea whispering noticable on 2 meters from the speakers. Any idea where this comes from? It is from both the two speakers. Tube problem?

Thanks in advance!