Another possible noise for mermen! Catfish make this sort of growling sound by grinding their bones together, it makes a very unique sound. Could possibly put some spin on that for creatures
Ooooo, that's cool! I looked it up because I wanted to know which bone and it's the pectoral fin spines rubbing against another bone!! I did something similar with the Merman story. But it was more the spines rubbing against one another or his scales.
But an actual bone being rubbed is a cool idea!
I love our animals and the creepy, weird things they do!