The air con is lovely and cool when the dial is turned to its coldest setting. I may be wrong in saying this, but I'm sure when I accelerate hard from say 50mph this "tut" noise happens and I'm sure I can feel the steering twitch slightly - as if a belt (timing, air con, cam - unsure) is slipping in the engine.

This noise is happening on the courtesy car I have too.......its a Zetec Climate. The air con unit on that makes a noise when left on when driving normally.