Back as a young Sailor in the 50's on a destroyer my job was to copy Fox
broadcasts. 24WPM coded groups 8-hours-on/8-hours-off forever and ever.
Bunk was below waterline, next to hull (5/8" steel, minor insulation padding).
Docked in at Norfolk D&S piers amongst many other destroyers.
Hit my bunk exhausted after overnight watch. Just zoned out and dozing off, and start hearing Morse from the hull. (Radioshack is 4 deck away, so this code isn't from there.) "Damn, this stuff is in my head. I'm going nuts."
Decide to take a walk, clear my head. Code goes away.
Lay back down, and it starts all over again. Morse, coming right in through the hull, but it sounds shaky and hand keyed. WTF? I really am going nuts!
Didn't say anything, finally fell asleep, and when I woke it was gone.
About a week later, same old thing all over again.
To make a long story short, turns out the Sonarmen had a system called "Gertrude" which they could key their sonar and talk to each other and to submarines when doing exercises. Every Wednesday morning they had a practice drill among all the ships in harbor.
73, de Hans, K0HB