I used to be a shipkeeper on a certain privately owned ex royal navy frigate.
Each night i would do my rounds on the upper decks and at a certain time each night I would hear what sounded like water tight hatches being closed and dogged.
The sounds would run from the bow to the flight deck, getting louder as they moved to the stern.
Thing is. I was the only person on board and when I would open the ship up in the morning all the hatches would be latched open....
I spoke privately with the other shipkeepers and they all admitted to hearing the same thing.
None of use ever ventured below decks at night and we were under strict instructions never to mention it to the public.