What is that crackling sound?

Over the past several days, we've been hearing a crackling sound in our hull, the other day the bilge pump ran for a while in the middle of the night. It ran the other day and I put my head in the bilge to look, which I do regularly before and after a sail. Remember rule number one is "keep the water out of the boat!" So as I'm laying on the floor with my head hanging down, I hear snapping, crinkling and popping sounds. I'm wondering do I have electrolysis issues, a short somewhere? If that the sound of water trickling down? It seems louder at night, maybe because all the noise of the marina calms down, no planes flying around etc. I've heard it before in Morro Bay, but I thought that was the ripples on the hull from the high winds. But what is it now? There's no wind. 
So being a good sailor I exhausted all of my faculties and Googled "crackling sounds in the bilge" and discovered I was not the only one who was tormented by this sound. My response was pretty tame compared to some who disconnected their batteries and in the complete darkness looked for sparks or leaks. Watch the video below to be amazed. A fiberglass hull is acting as a speaker for any sound under the sea. We've heard sonar from submarines resonating through the hull.