k, this is a random issue. While listening to headphones on my MBP, I have noticed a low hum when my computer is plugged in. However, when I unplug my computer, it goes away. And here's the really weird part. When my MBP is plugged in, and I touch any metal part of the computer, the hum goes away. But, if I take my hand off the computer, the hum returns. Could there be some sort of power leak, grounding issue, or something weird going on with the computer?