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.
Ground loop on the analog inputs.

Try disconnecting all your analog inputs and add one at a time. See if you can recreate this with either alone, or if it requires both.

Try plugging everything into the same strip, and unplug any coaxial cables involved, such as cable tv/internet if they are one of your inputs.
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i'll try it, thanx. everything is plugged in the same power conditioner, and no cable box or tv in the room.
I spoke with McIntosh about this with my tube integrated amp. They said it was normal.  But I don't get a louder hum when the volume is increased.  It stays the same low level hum
Low level hum in McI is wrong situation, its not normal, Probably, You have problem with el. capacitors in power supply. 
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I just checked and the low level hum is not increasing with volume. I don’t know why I thought it did yesterday. Why aren’t the digital inputs creating that hum? Why only when one of the three analog inputs is selected? Even without any interconnects on those analog inputs....
Hi iasi!! 

This probably has to do with how the switching is being done internally. Sounds like you may have an analog ground loop, something which digital inputs are very good at ignoring.

Did you try removing all analog inputs, then adding them one at a time?

Is the hum coming from the speakers or the amp? I chased a loop for almost a year. I tried everything from power conditioners to low noise Power supplies (I have electrostatic) to noise harvesters.
I finally found the hum.
It was coming from an old bug zapper hooked up in the garage. It was on a different circuit, I can’t explain it. When I unplugged it to do some maintenance I noticed all my ps audio noise harvesters stopped blinking and my hum was gone. It must have been putting dc on my line. My amps are Mcintosh too. 
Bad filter cap in the power supply could be at fault. IMO
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i unplugged everything from the int amp...turned it on and the faint hum is present whenever one of the three analog inputs is selected...the hum disappears when one of the 4 digital inputs is selected...i changed the power cable for the int amp to the one with 2 prongs...no difference...anyway, it is not that obtrusive when the music is playing. i was just trying to understand why is the hum present only on the analog inputs and not on the digital ones.
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Yes, if it's only a question of which input is selected, even if no component is attached to that input, then it obviously sounds as if the problem is internal to the amp, especially as digital input is quiet.  Hum on any/all of the line-level analog signal inputs, then?  What make of amp?
yeap, a faint hum on any of the analog inputs...it doesn't go up with the volume...but it's there...Gold Note IS1000. Not planning to replace the amp since it is the most musical one i've had in my system...
If the hum is coming from the speakers and not from the unit itself, then it is likely a ground loop.