if i have my phone plugged in, and i stroke the metal back of the phone in the right way, i feel a kinda subtle vibration??

is this:

1. the smooth surface of the phone rapidly sticking/unsticking to my finger, like how dragging a chair leg across a tiled floor makes a squeaking sound?

2. the vibration motor in the phone?

3. 50 Hz AC current leaking through the USB phone charger into the capacitor that are my fingers?

4. ghosts???

5. a joke??????