Actually, it's not there when I "touch" the knob. It's only when I "turn" the knob that I can hear it. It's very tinny, too. I can't hear it in most of my music... except for classical music when the room just suddenly dies off (completely) and I'm left with absolute silence.
If anything, I noticed it when it was midnight... and everyone was asleep. The ambient noise here right now according to my meter is approximately 32dB... so you can pretty much imagine how quiet it is at night.
And it's not there when the computer is not plugged in. Which is weird. I have tried different plugs, and even went to Starbucks a little while ago to test. It was just the plug.
It happens with my iPod Video, MacBook Pro (the Retina model), and my Vaio TZ, so I don't think it's the device.
Funny enough it doesn't happen to my phones (Galaxy S3 and iPhone 5).