4 year old cat started making new noises out of the blue, why?

My cat (Sunshine) is a 4 year old spayed female grey tiger/orange tabby mix. About 2 months ago, we adopted a (then) 4 month old (Sirius) neutered male black Siamese mix. Sirius is super vocal and will meow all the time. Sunshine purrs loudly, but over the past 2 weeks she has been making this weird squeaking noise sometimes, while purring. I watched several videos of cats making noises, and the noise is apparently called a trill. Any ideas why she would be making this noise 4 years into her life? She was raised with dogs until last year, and had never seen another cat, after she left her mom, until we got Sirius. Did she just learn how to be a cat?