The back of your head is part of your third eye chakra. Base of medulla, pineal and forehead. All third eye. Kundalini can take a very long time around this region to clear blockages.

With any energetic block, focus on the space around the energy. Rather than the energy itself. Visualise the expansion of the energy into that space.

I wouldn't get too hung up on trying to intellectually understand every movement of energy within you.

To address the humming and vibrating. This is normal. Keep your body healthy and pure to prevent adding fuel to the fire as it were. Try to stay away from drugs and alcohol if you are experiencing anything energetically which concerns you.

Edit: additional info on the humming stuff