I think they sound a lot like a glow engine. Regarding noise, prop size makes a very noticeable difference as one might imagine. These little guys make quite a racket when turning an 11x6 close to 14k rpm while you're breaking them in. Moving to a 12x6 and then to a 13x 6 as the engines broke in made noticeable differences at each step. Nothing official to point at, but with a 13x6 I don't think they're quite as noisy as a 20cc engine w/stock muffler.