I've formally studied three natural languages. I've casually studied dozens. The number of languages I have personally designed, at least in part, probably also tallies in the dozens. I'm currently working on a language family. Portions of any of them can and have fallen out of my mouth like strangely shaped freshwater pearls. I have a button that says, "Please tell me when I'm not speaking English." It won't be long passages, usually, but individual words or phrases, yes. A lot. But that's not the part that throws people.
It's the language bits that aren't exactly words, but have meanings, that throw people.
Housemate: "Did you just make a sound like a Wookie?"
Me: "That's an interrogative trill, it's from Waterjewel. It means "Who's there?" or "What are you doing?" or ...
... yes. I just made a sound like a Wookie."
Specifically, these are trills made far back in the mouth and throat, with tongue and soft palate. The interrogative trill can stand in for pretty much any generic query, and it has a rising tone. The declarative or commentary trill has either a level or a falling tone, and can mean things like "I'm here" or "uh-huh." I picked these up from my Waterjewel characters, long ago, because ... well, I'm linguistic SillyPutty. I mimicked animal sounds and objects when I didn't have human languages to fool around with. So it wasn't that much of a stretch for me to pick up the trills. I hear them a lot from my characters, particularly since the ones who can travel to me will use the interrogative as "Are you busy? Can I come in?" So I started making the sounds out loud, hm, must've been junior high or so.
Some of the odder sound effects I used in conversation when I was younger have faded. I can still make them, but I don't make them with other people anymore. I still have the ability to hear and learn new phonemes, though. My language acquisition window only got partway closed before I wedged it open.