You can tell why domestic cats make a trilling sound by observing the circumstances under which they make it. I can think of two situations but there will be more.
Undeniably the trilling vocalisation is a call which is made at a close distance and it is friendly. Those are the two key elements for this domestic cat sound.
But some cats hardly make the sound at all. Like all things feline there is a lot of variation between individual cats. It is fair to say that some individuals will never trill while others trill all the time.
This may be linked to the whether the cat is outgoing and confident or shy and timid. The former personality is more likely to be behind the trill.
My neighbour’s black-and-white female cat visits my home at night quite often. My cat sort of likes her. He plays with her but he’s too rough. As she approaches the cat flap to come inside she starts trilling. As she enters the home she trills. As the wanders around the home at 3 am she trills every ten seconds. What does that tell you about the purpose of the feline trill?
It can only be one thing: it is a greeting call. It is like saying, “Hi, I’m here. How are you?”
Close call on wake up
There is a video of a cute ginger male cat who trills when he is woken up. His photo is on this page. The woman making the video deliberately wakes up her cat to film the trilling. He is consistent in his trill.
The fact that he trills when woken indicates to me that the cat is greeting his owner. He is suddenly presented with her when he wakes up and says, “Hi, why the hell did you wake me up!?”
I am just joking. He’s simply greeting his owner as he suddenly sees her.
Variations and modified meanings
It is likely that there will be variations in the trill and variations in its meaning and usage. Once again it is an individual cat trait. Feline vocalisations are very fluid.
It is wrong to pigeon hole cat sounds into clearly defined categories as they can merge a little bit or a lot.
A secondary purpose is to attract attention. But then a greeting does that automatically.
Expression of contentment
Greetings are also linked to contentment. If a cat is greeting another or a person in a friendly way in using the trill they are going to be happy. The trill is a sound of happiness or elevated mood I’d suggest.
The Humane Society of America say that the trill is first used when a mother communicates with her young offspring to attract their attention.
It seems that Maine Coons are predisposed to trilling. One of their charms.
The trill is made with the mouth closed. There is no visual change in the appearance of the cat’s face.