Pictured is a barrel adjuster. There is one on the right side of the bicycle for the rear shifter and one on the left side for the front shifter. This Instructable concerns only the barrel adjuster for the rear shifter. I have had most of my problems with clatter at the gear cluster on the rear wheel.
The cure for the clatter problem is really pretty simple. I had read a very good manual shortly after getting my new bike with combination brakes and shifters, often called "brifters." Even though the manual discussed these problems, I forgot and had to relearn what I had already read. I was almost ready to accept some shift problems as "normal."
Step 1: First step
Release the tension on the shift levers and derailleurs so the cables are in their most relaxed positions. Loosen the cable clamp (hex socket screw between my two thumbs). Pull the cable to hand tight and tighten the cable clamp screw as tightly as possible.