I'm not all that sure what the part is called or what it does but looking at my own it appears that it sits in a machined recess in the slide when in the 'normal' position and would be depressed by the slide when it moves rearward. If you shake the gun with the slide in place does it make the noise and then the noise goes away if you lock the slide in its rearward position and shake it? Seeing that the slide bears on that part when it moves rearward I believe that would be the case..