Faint hum....

I've been struggling to understand this issue for a while...my integrated amp has 2 RCA inputs, 1 XLR input, and a few digital inputs (Toslink, Coax, etc). When I turn the system on, i'm usually on either RCA1 (CD player) or RCA2 (phono stage connected), there is a low hum in the speakers. If I increase the volume, it gets louder. If I mute the amp, the hum goes away. It's noticeable, but it doesn't bother me when the music plays. What I find it somewhat strange is that it has the same volume if i switch between RCA1 RCA2, and XLR (I have nothing connected to this input!!!). But when I switch from one digital input to another (Coax, Toslink1, Toslink 2), there is no hum. Everything is dead quiet.
So what's going on? Is there anything I can do to eliminate this hum? I tried all sorts of interconnects, speaker cables, the hum remains when I switch from one analog input to another. It might have different intensities, but no cable got rid of it.