How do some catfishes produce humming or growling sounds?

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knifefish — ostariophysan Knifefish, or featherback (Xenomystus nigri)

In several catfish families, humming or growling sounds are produced through a specialized springlike mechanism derived from modified portions of the fourth vertebra. Muscles extending from this mechanism to the skull contract, causing the springs and the wall of the swim bladder to vibrate rapidly.1

Additionally, some other catfish species generate sound through the rubbing or grating movements of their dorsal and pectoral spines.1