2002 ford expedition clicking sound and won't start?

I had a similar problem. The rear hatch area was making an intermittent clicking noise. The battery went completely dead. I attached a charger and it wouldn't charge the battery, came up with an error code saying my battery was bad. So with a booster battery attached I could still hear the clicking. I popped the rear glass window using my key fob and heard an electric motor run briefly. I closed the hatch, no more clicking. I was then able to charge the battery. By the way, the entire rear hatch is coated in ice and it's about 20 degrees out.