Originally Posted by cyccommute

Your derailuer is out of adjustment because of cable stretch. It is probably making noise when you shift and when you ride but you don't notice it. One of the other posters was close but his advice was a little off (the other one is just off). Put the bike up on a repair stand (or hang it from a roof rafter). Shift it all the way to high gear (i.e. the cable should be in its most relaxed condition). Gently pull on the cable where it runs next to the downtube. It should be just slightly tight with a small amount of play in the cable. It shouldn't pull out from the frame too far nor should it be as tight as a guitar string. If it is loose, you should be able to take the play out with the derailuer adjusters and/or lever adjusters.

Shift to the next lowest gear and pedal if forward, not backwards and run it through the gears. If adjusted properly, the gear train should be relatively silent. If it clatters or makes popping noises or if the chain appears to skip on some cogs, you need to adjust the deraileur. Turn the cable adjuster on the derailuer to the left (towards the wheel) to tighten it and to the right it loosen it. Work in small increments, a quarter turn at a time, and the pedal it again. Shift gears often (up and down) while adjusting until the bike starts shifting as you like. Sheldon Brown, as usual, has a good indepth explanation of how everything works and how to make it work like it is supposed to.

Hope this helps,
Stuart Black
"Dancing chains, good. Skipping chains, bad."