Bicycle Guide Pulley
The upper derailleur pulley (called the "guide pulley" or G-pulley) pushes the chain to the next sprocket. A proper gear adjustment aligns the guide pulley under the intended sprocket.
Changes to the inner wire tension causes movement in the derailleur. The derailleur body is fitted with a spring that is pulled tight, or relaxed, by the inner wire. Pulling the inner wire moves the derailleur cage and guide pulley in one direction and tightens the spring. Relaxing inner wire tension allows the spring to move the body and pulley in the opposite direction.
It is normal for a chain to make some noise during a shift. The shift may appear subjectively "noisy," "loud," or "rough". Factors like the type of chain or sprocket, the wear on each, and the amount and type of lubrication will affect the noise a chain makes during shifting. The limit screws typically can do nothing to affect the noise during a shift between cogs. Noise from the chain as it rides on the sprocket is, however, a useful symptom. There is for any given bike a "base level" of noise from the chain as it passes over the sprocket teeth. When the derailleur jockey wheel is out of alignment, the chain may make excessive noise. To demonstrate the "base level" noise, shift the bike to the second sprocket by pulling the inner wire. Continue to pedal and move the inner wire slightly to hear changes in the level of noise. The quietest level of noise may be considered the base level for that bike.
For Additional Information see Park Tool's Rear Rerailer Adjustment page.