Buzzing Fix, but why does it work and what's the underlying problem?

I seem to be having the same buzzing noise everyone else is talking about.  It comes from the "joint" of my 8600 where the LCD meets the back of the keyboard.  It's mostly evident when using the touch pad, but is intermittent when the computer is just sitting there.  It can be heard on or off AC power.

I have a USB logitech wireless mouse.  When I plug the USB receiver into the USB port.  The buzzing noises go away COMPLETELY.  Why does this work and how can it be fixed without constantly having a USB device connected?