(Sorry if this has already been a topic.)
I thought echolalia was a very strange thing when I first heard of it, and certainly wasn't something I do. Until I read that it doesn't have to be out loud -- that repeating your own or someone else's last words in your head over and over is a form of echolalia.
Annnnnd I do that. Anybody else?
With me, it just plays in my head like how a song gets stuck. I'll usually divide whatever I'm thinking of -- often parsed down to a single word -- down into letters, and sort of tap it out with my fingers and try to make it come out evenly (ending on an end or index finger). Like the word "computer" -- I'd use my index finger for c, middle for o, ring for m, pinkie for p, ring for u, and so on until it ends up at my thumb. Rinse and repeat. It's a pretty unconscious habit, and with tricky words I'll play around with it for a while to make it "even." I like nine-letter words; they work really well.
With sentences or phrases, I'll just repeat them over and over in my head as I go merrily along. It doesn't happen in the middle of conversations 'cause I'm too busy figuring out what I'm supposed to say next. If it's something I said, I imagine it with all sorts of intonations and whatnot. I never really realized this was a "thing," but so it is with most Aspie-type traits.
(I'm not officially dx-ed, so take this with a grain of salt. It's always possible that I'm just individually crazy.)