I have this problem and it appears to come from near the "magsafe" power connector. The magsafe connector was quite hot to the touch. Two of the connecting pins are damaged due to overheating. (When a power carrying connector doesn't make good contact, you can get a lot of heat.) When I remove the charger cable, the sound goes away. My plan is to replace the small circuit board (Magsafe 2 DC-in board) in the MBP because the contacts are damaged. If I'm right, that will fix it. The Apple people say to always make sure the connectors are clean so they make good contact. Ed