This actually started just after we bought the boat, but I haven't managed telling the story until now. When my wife and I bought the boat, we didn't actually stay on it until after we moved it to the marina at Hammock Beach.
Well, the first night at the marina we climb into the berth and turn off the lights. It was a good temperature so we didn't have AC, heat, or fans running. In the quiet of the dark, we started hearing a crackling sound. We both thought it was a bit strange and just a little concerning. In our limited time staying on boats, we had never heard anything like it so I got up and listened intently trying to locate the sound. It appeared to be coming from the lower hull. Not sure what the sound was, I pulled up the floorboards to look into the bilge. I didn't see a thing out of the ordinary. With the floorboards up, I once again listened and the sound seemed to come from all around on the hull. And with the floorboards up it sounded a bit like the fiberglass was cracking.
I sat there and listened, tapped on the hull (which I know is about 5/8" solid fiberglass at the bottom) and pondered what the noise could be. Is the somewhat cool water causing the fiberglass to expand and contract unevenly? Is something eating through the hull? Is this just a "normal" sound that I've never heard before? After staring at the dry fiberglass hull for a while, it was getting late and I decided that the boat wasn't sinking so we just put the floorboards back and went to sleep. Hopefully the rush of water will wake us up if the hull gives way. The next days the boat remained afloat but the crackling sound could be heard anytime the boat was quiet.
Now, fast forward to a week or two after I get back to the boat. I still hear the noise but, while I have no explanation for it, have dismissed it as a "normal boat noise". Talking to one of my dock neighbors, they were telling me how they were hearing a noise like crackling or water leaking into their boat. They invite me to come listen and, sure enough, it is the same noise that I've been hearing all along. They hadn't noticed it until they started doing some work near the hull (they are a motor yacht and their hulls are covered with cabinets and other things that mask the sound). I told them I had been hearing the same thing since I've owned the boat and had dismissed it. They couldn't believe it was the same noise so I invited them to come listen. Sure enough, they agreed it was the same noise.
As we were standing on the dock discussing the strange sound that we both hear, one of the locals wandered by and offered an explanation. Apparently, when it gets a bit cooler, some small shrimp tend to come out and they are the cause of the noise. Neither of us had every heard of such a thing, but sure enough, a quick Google search reveals it is quite common.
Or for an over-dramatic video explanation...
In any case, it is fascinating...and good to know that my fiberglass boat isn't dissolving.