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I recently purchased a Boston Acoustics Digital Media 2.1 system w/ powered subwoofer.  When I plug in the power cable to the subwoofer and switch it on, I hear a constant, high-pitched squeal.  The squealing changes frequencies when I plug in, say, the control module.  But when I turn the power switch off and unplug the power cable, I hear the frequency gradually get higher and higher until it completely stops, maybe a minute after cutting all power to the sub.  I've tried moving the sub to a different outlet, but that it still emits the squeal.  I though it might be a problem with the powered subwoofer, but other two-pronged devices in my room make similar noises.  I have an xbox 360 battery charger that makes a ton of noise when plugged in.  Even some of the three-pronged cables, like for the tv and xbox, make a small amount of buzzing noise.  Do you think this is just a grounding issue?  I live in an apartment, so I don't know what I would do to verify this.