A stereo, a loud hum, and three Macs

Audio/hardware/electrical issue.
Friend has a brand new Mac Pro, a G5 tower, a MacBook Pro and a killer classic stereo. As of a month ago, he could plug in either the G5 or MBP to the stereo via mini-jack and get great sound from iTunes.
Then something changed, and suddenly the G5 started generating an overpowering bass hum, but the MBP continues to work fine. The new Mac Pro also generates the same hum.
In researching, I suspect it has something to do with the 60 hz ground loop, and I know there are ground loop him eliminators out there, but I don't know enough about this topic to pick a good solution. Computers & stereo are plugged into the same circuit via grounded outlets.
Three questions:
1) what could have changed to cause the G5 to start to hum, after years of working? audio cable is the same. Surge protector is the same.
2) why does the MBP continue to work fine? Something to do with running of a battery vs. a power supply?
3) what's the best way to eliminate the hum in this situation? The stereo is "old" and does not have optical inputs.

If the stereo has a two prong plug, try reversing the plug. It it has a polarized plug (one wide prong) than can't be reversed, try a different outlet.

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