I've become an electrical conductor for my headphones... but how?
The "buzz" is almost certainly RF pickup. This can be the mains (low frequency, 50/60/100/120Hz, unlikely in this scenario given a phone is involved) or rectification of the carrier frequency of the phone - remember how old tape players would make a "duh duh, duh duh" sound whenever a text message is received.

Human bodies are conductive. When your skin is dry you are about 100kOhm to 1MOhm. When your skin is wet, that goes down to about 10kOhm to 100kOhm. So there is nothing unusual about becoming a conductor.



As to why touching a metal keyboard kills the hum, or even touching the headphones jack. Basically as you are a nice large conductive area. When connected either to earth (e.g. via the keyboard shell and PC) or to the ground of the phone (e.g. headphones jack), you are basically changing how your body is interacting with the electric fields around you.

EMI is a dark art - it's hard to say exactly what current path you are interrupting, or how you are interacting with the fields. But generally if you ground yourself it stops you acting as an antenna - sort of like shorting yourself out.



One simple option you could try is to buy an anti-static wristband. That would allow yourself to be connected to earth (or to your keyboard) without having to touch the keyboard. Alternatively you could try simply charging your phone from something other than your keyboard to see if that stops noise injection.