Electricity coming through screws, USB, and headphone jack on 2009 MacBook
If you are using a grounded plug and feel ground loop current you should either have the outlet serviced or the computer serviced.

It's normal (and safe) for ground loop current to exist and feel lightly tingly on Macs with ungrounded power supplies since there are spinning fans - especially inside the thermoplastic white Macs. It's also almost inconceivable that the low current DC voltage that a Mac battery and power adapter provide could be a safety risk, but it could be a sign of internal damage and worth having an expert check out the wall socket and or the Mac plus it's power adapter.

Since your comment reinforces the level of detail you are seeking, I would say that any Apple repair shop that received that Mac for repair might use a multimeter to see if they can sense an voltage on ground pins and/or the case. They might place a new DC in / magsafe board in and re-test to see if that's the cause. They would of course test with their adapter and then remove the components in the main unibody section to look for loose screws, scortch marks or other signs of burns or liquid damage to the components that might cause this abnormal current situation:


battery
rear vent
fan
optical drive
logic board


Once you've gone over these - hopefully you can visually see the problem or remove whatever is grounding current / voltage to the frame.