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I had some fun with this at a local chain store selling computers. I could feel the same thing in a couple laptops on display. When one of the "geeks" came by to help me, I showed him what I was doing and asked him if he could feel the electricity, too. He did. Then I said "I think you have an electrical short in your power somewhere and something's not grounded right ... and if you unplug the power this will be safe again." (I specifically used those words to imply this is scary stuff). He unplugged it and behold, the problem went away. He plugged in back in and it was back. Not all the computers did that and I explained that the plastic ones were safe when something's wrong with the power. He got the manager and showed him, and the manager shut all the computer displays down. I managed to hold off laughing until I returned to my car. I haven't been back there since.

This is a common problem with computers and other devices connected with ungrounded wall warts. The cause is that the adapter has a transformer that is not shielded and the secondary is not grounded. It CAN'T be shielded/grounded because these adapters usually do not have a ground pin. The end result is the secondary, and thus the DC, is floating between 0 and 120 volts. Odd as it may sound, connect it to 240 volts (be sure the adapter is rated for it ... but most are) to fix it. Then the system will be floating between the two hots. Unless you are on 2 legs of three phase, the buzz should be virtually gone. If you are on three phase connecting it to 208, at least the buzz will be reduced.

Another fix is to wire up a transformer to give you a split phase 60/120 system, with the secondary center tap properly grounded. You'll have 2 hots at 60 volts, but that will be 120 volts between them. If you can find an UN-polarized 1-15R somewhere, use that so that polarized plugs can't go in (most computer power adapters have unpolarized 1-15P). This would be unsafe for some devices such as lamps with screw bulbs since both conductors will be hot (60 volts is still somewhat dangerous).

Read up on NEC 647 so you know what kind of power this is.

Personally, I'd prefer the 240 volt method (ignoring 210.6(A)(2)) as that also gives me brownout protection.

FYI, I can feel the 60 volt potential on the metal surface parts of my laptops/netbooks when plugged in. I touch the surface very lightly and stroke the finger tip across and feel the 60 or 120 Hz vibration. It's the same frequency no matter the speed I move the finger. It goes away when on battery.

I had some fun with this at a local chain store selling computers. I could feel the same thing in a couple laptops on display. When one of the "geeks" came by to help me, I showed him what I was doing and asked him if he could feel the electricity, too. He did. Then I said "I think you have an electrical short in your power somewhere and something's not grounded right ... and if you unplug the power this will be safe again." (I specifically used those words to imply this is scary stuff). He unplugged it and behold, the problem went away. He plugged in back in and it was back. Not all the computers did that and I explained that the plastic ones were safe when something's wrong with the power. He got the manager and showed him, and the manager shut all the computer displays down. I managed to hold off laughing until I returned to my car. I haven't been back there since.