Jazz I'm thinking of buying has a transmission hiccup when starting from stationary

I'm looking at a 2007 Honda Jazz, took it for a test drive, but when speed up from still/very slow there's something a bit weird with the (auto) transmission. Not quite a clunk, and not quite a grind, and once it gets above 20km/hr, there's not any problems. Any ideas what it would be, and/or how difficult/expensive an auto transmission is to fix?