I've got a nearly 3-year-old MacBook Pro 15" 2.16GHz (MacBookPro2,2). When I have am not using the extended, grounded adapter for the power supply, just using the simple, two-prong plug I can hear a buzzing when I use very sensitive earbuds. This goes away if I touch a metal part of the laptop.

Also, I can feel a weird, fuzzy feeling when I brush the metal parts of the laptop lightly with my fingers/skin. Somewhat similar to feeling of a touching hair or a balloon that's charged with static electricity. But I'm not getting sparks or anything. And if I'm touching a metal part of my laptop solidly (not just brushing it) and then I touch someone else's skin I can feel the same effect and so can my victim.

I've noticed similar effects with an ungrounded electric blanket. But with that the buzzing can be easily heard without headphones.

Is this a defect, normal, or something else? And what exactly is happening?