My brother has two engines with the Milodon gear drives. They do make a small amount of sound, but you only really notice it at idle. I can hear it because I know to listen for it, but I bet most people don't pick up on it. Once you put the car in gear and drive away the little sound it produces is drowned out by the exhaust.

I would buy a Milodon or just stick with a nice chain setup if you don't want to spend the $$$.