Once upon a time I used to work in a group of submariners, including sub skippers. One story they told was recording the clicking sound of a school of snapping shrimp 🦐 and overlaying that sound over voice communications when transmitting. The receive submarine also had the shrimp 🦐 overlay in the receiver so the voice message could be recovered by removing the overlay. That’s back before spread spectrum communications.
https://youtu.be/rU_gKrKWECQ
https://youtu.be/rU_gKrKWECQ