Trill (vocalization)
A trill was a vocalization made by various animals. It was a vibratory and quavering sound.
In 3188, the trance worm Molly trilled after approaching Cleveland Booker, who responded by touching her lowered head with his right hand. (DIS: "That Hope Is You, Part 1")
The closed captioning for the episode identified this vocalization.
In animals, such as cats, trilling was "seen as a sign of their comfort, ease, and trust" in an individual. [1]