What can cause a very low frequency sound from a laptop?
This is coming from an internal DC to DC converter.  I have a Dell XPS laptop that produces similar high pitched whine and/or hiss under certain conditions.  These converters can operate in several different modes.  One of these is constant on time, where the converter pulses on for a constant period at a variable frequency.  It's possible that the LED provides just the right amount of load to get the converter switching in the audible range - LED off results in switching so infrequently you don't notice and computer on results in switching well outside the audible range.