Avoiding electrical shocks/statics
I had a similar problem where my Mac Mini Late 2011 was giving me vibrations when I run my fingers over the aluminum part of the body - also known as "humming current". This type of current is very feeble and does not result in any shock to the body or cause any harm to the hardware components. But it is annoying. I zero-ed in on the faulty grounding of the electrical outlet. I had to run an extra grounding wire to discharge extra / spiked voltage from my power outlet into the fan loop made of iron in my ceiling. After that the problem is solved.