Sometimes it is current leak from other sources, eg. when I plug in an external wire to the soundboard, I get stings, or when I plug direct to a LCD projector, more stings.
Just avoid touching the metal screws or other metal parts. Stings doen't stop even when cam is powered down, as long as connected to external appliances with power up.
I now use gaffer tape to cover up all the sensitive areas to prevent stings.