I agree with everyone... it is alarming at first, but this is normal for the lens. I've heard this sound from the first day and almost 3 years later, it has not resulted in any kind of malfunction. My lens gets fairly heavy use... thus, fear not.
600 TC does the same. It freaked me out when I first got to use the lens. The 600 is a tank though - its seen some (unintended) abuse and has held up great so far. I would expect the 400 TC to be identical in this regard.
