What is the buzzing sound I hear via headphones when I touch my charging laptop?
it's connected via 2-pole unearthed - 45W MagSafe 2 Power Adapter,
which is connecting to a 3-pole earthed power strip

Power adaptors of this type need to pass conducted emission testing on their DC output wires and, in the absence of an earth pin, will normally connect the DC output to the live DC side of switching flyback transformer via a low value capacitor in the range around 2 nF. Because of the way that bridge rectifiers work, this couples a small AC current from your AC supply to the normally isolated DC output.
There is no danger with this level of current but, it can flow through audio circuits due to body capacitance when touching metal parts of the laptop. I suspect that this is the problem.
You can sometimes feel a slight tingling when you touch metal parts of the laptop. This is another indication of the same thing.